Mostyn Cartwright is a character appearing in the episode Send in the Clowns of the ITV crime drama Midsomer Murders.
Mostyn had a very troubled childhood as he was charged for several counts of armed robbery and aggravated assault, earning him a "Youth Offender" record. Though despite this, Curtis Ferabbee, the owner of the local farm in Swynton County, saw potential in him; remarking him as the "son he never had" and hired him to work for the nearby abattoir, where he slaughtered the livestock for meat products using a specially-patented "automatic gas chamber," which he would deliver across town. He trusted him to the point where was given copies of all the keys in Ferabbee household, though Curtis also used him for a plan to swindle his brother out of the land he built his circus on. Part of the plan was having Mostyn dress up as a clown and go around stalking people in Swynton; along with targeting specific people by sending them ransom notes, written in pig's blood. With enough fear and division among the locals to pin it on Big Top, Curtis would've been able to reverse the land claim with a fair share of it going to Mostyn. He even went to the extreme of trying the destroy the circus through arson as he would collect samples of oil and pour them into a bottle that he would've used to light the flames. Unbeknownst to Curtis, Mostyn was a merchant for a black market in "black pudding" and would go into a basement room in the cellar, off-limits to everyone except himself, where he would dry pig intestines and deliver them to anonymous buyers. On top of that, Mostyn liked to steal every now & then and was confronted on this by Sophia Bellini, though chose not to report it as she didn't want to compromise his job. As for his personality, Mostyn is known to be quite rude, cheeky, and sometimes disturbed. He has also mentioned having a pet ferret and that he wouldn't feed him one of Mr. Denton's "organic steaks."
On the first day the circus comes to town, Mostyn is first seen in his clown makeup terrorizing a teen walking home from university and soon enough, CCTV footage of him becomes available to John Barnaby and Jamie Winter who are on the case. At midnight, Mostyn breaks into a substation and causes 300 blackouts all across the Swynton area. After taunting patrons at Denton's Pub & Grill from outside the window, he uses the opportunity to break into Daniella's belongings; among them, the locket that belonged to Elsa and took it back to his cellar. He also used the opportunity to place a rag soaked in paraffin under the bleachers so he could come back and set it on fire. The next day, we get a proper introduction to him where he attends a town hall meeting regarding the clown, blaming Ashley Denton for not doing anything and accuses him of paying off the circus, which Denton can only respond to by bringing up "animal husbandry" affairs of his. When its suspected that Curtis' rifle has gone missing during the murder investigation, Ferabbee tells them that Mostyn may have had access to them as John Barnaby and Jamie Winter go on to interrogate him. After giving them a quick tour of the factory, Mostyn leads them off and vows to let them know on any updates about the missing rifle. As soon as they leave, he heads all the way down into his secret basement and opens up a drawer full of rotting pig meat that, among it, is a sniper rifle that may have been the one stolen from Curtis. At midnight, Mostyn, in his clown suit, throws a brick through the window of Denton's restaurant; carrying a note to him in pig's blood that reads "WHOSE SIDE R U ON? TIME TO CHOOSE." as he menacingly waves back to Ashley; addressing the irony of him conspiring with Curtis to overturn the land while also funding the circus.
The next day, Barnaby and Winter are on their way to investigate the "window smashing" when they spot "clown" Mostyn standing on the other side of the road, leading Winter to give chase. He gets away, but not before leaving behind a shoe that the DCI takes in for evidence. Sometime later, Mostyn pops by the fence walled around the circus field to spot Curtis talking with his nephew about a planned project he promises to invest if he makes his Dad give up the land; building up a bit of jealousy in Mostyn as Curtis pats Harry on the back. When its suspected that the sword that killed Ashley was sharpened on an industrial whetstone, their first instinct is to check in with the abattoir where Mostyn defends that there are other services in the area and that he wouldn't wish harm on the circus; who is one of their biggest customers. Later, in the middle of nowhere, Mostyn is seen delivering three packages of fermented pig blood to the driver of a passing-by car. Meanwhile, he delivers a note to the Ferabbees, Joe and Harry, that reads "GO NOW OR UR NEXT." While possibly intended for Joe, it could've also been directed to Harry as he looked displeased to see him bonding with Curtis. He then goes back to the abattoir to check on some things while, unbeknownst to him, Daniella is hiding inside the building and is aware of him stealing from their wagon. As soon as he's gone, she opens the cellar door and takes back the locket; triggering the alarm at 16:58 and grabbing the attention of Curtis who is accidentally knocked into the gas chamber, killed, and placed by a robotic arm to hang among the meatbags as discovered by Mostyn upon coming to work the next day.
At the scene of the crime, Barnaby and Winters get Mostyn to explain the rest of the gas chamber's mechanisms, along with how the alarm system works and who manufactured it. Barnaby objects to the lack of human involvement, though defends that Curtis was all for the technology that ended up killing him. When he brings up the possibility of an alarm only going off when there's an intrusion, Cartwright states that they've been dealing with vermin problems lately and that the motion sensors aren't set if the Ferabbee's aren't home; except in the office and store room. When Winter winds up stepping in a puddle of oil on the driveway during their way out, an anxious Mostyn appears in as he had also heard him bring up past criminal charges they found of him. Fearing that the cops may find out, he flees to an undisclosed area near the forest where he burns the rest of his clown costume, though having gained enough clues as to what he was covering up (Sophia's confession, the delivery schedule, the sack, the basement, pig's blood on the shoe), Barnaby and Winter corner Mostyn who threatens with the sniper rifle. There, he confesses to the plot Curtis involved him with, the black market, and the attempted arson; the latter of which failed as he bragged to Ashley about it, who tried to remove the rag the on the day Terry Bellini was shot. Winter is able to calm Mostyn into laying down his gun, though when asked if it was the one he used to murder the other clown, he defends that it's his own rifle, only uses it for vermin, and adds that he would never betray the only man [Curtis] that had any faith in him. As police sirens wail in the distance, Mostyn looks down in defeat as he may possibly lose his job for good; along with having another charge added to his "Youth Record."