Midsomer Murders is a difficult programme to fathom. At best, it’s an anachronism; a show cut adrift from its spiritual roots in the gentrified ITV police dramas of the ‘80s and ‘90s (Inspector Morse and The Ruth Rendell Mysteries, for example) and caught in a deluge of better, more contemporaneous detective-based shows. At worst, it’s a patronising, confusing, cliché-ridden mess where low and high culture are uncomfortably thrown together – rather like the private school at the centre of this episode, the first since the departure of John Nettles.

Nettles’s DCI Barnaby is replaced by Neil Dudgeon, playing another DCI Barnaby, whose first case in Midsomer sees him and DS Jones (Jason Hughes) investigating a convoluted case of sex, drugs, vintage sports cars and – of course – a plethora of murders, all set against the background of a girls school on the verge of financial collapse. Struggling not to spot the rather obvious denouement too early, the two detectives raise their eyebrows a lot, smile indulgently at the behaviour of the schoolgirls, chat, take a dog out for a drive and eventually, two hours of tedious television have passed without Barnaby even having the chance to unpack his possessions or his wife from storage.

Given the recent controversy about a lack of diversity in Midsomer Murders, which has seen producer Brian True-May suspended for stating that having ethnic minorities in the cast ‘just wouldn’t work’ and proclaiming the show as ‘the last bastion of Englishness’, an episode set in a boarding school presents an opportunity to move away from the usual, exclusively white and mostly middle-class parallel universe the show inhabits. Perhaps not surprisingly, given True-May’s truly amazing comments, it fails to take this opportunity at every turn.

There are no perceivable black or Asian pupils at the school, and the only two working class ‘scholarship’ girls (played by Daisy Keeping and Gwyneth Keyworth) are treated with genial contempt by the script and the caddish Charlotte Cameron (Clara Paget) alike. Sixth-former ‘Charlie’ is a lacrosse whizz with a sideline in dealing coke at London nightclubs and an unerring knack for looking like a woman in her thirties dressing up saucily for a Back To School party. Although the three girls get the most amusing exchange that a depressingly humourless screenplay has to offer (‘You’re dead!’ ‘Bitch!’ ‘Snobby cow!’ ‘Your mum’s a prozzy!’), they have to be the most unrealistic schoolchildren seen on TV since Jeanette Krankie hung up her cap and blazer.

Even worse is Luke Allen-Gale’s DJ Dave ‘Doggy’ Day, who speaks the worst kind of clichéd sub-Tim Westwood patois. The moment he bumps fists with elderly retired racing driver Peter Fossett (David Warner) is so cringe-worthy it makes Gordon Brown’s banter with JLS on Comic Relief seem credible.

The climax of the story sees the new DCI Barnaby hanging above a threshing machine. It’s hardly a spoiler to say that the latest principal detective in Midsomer survives, but the outcome of Brian True-May’s suspension remains less certain. The fate of the programme he has produced for fourteen years must surely – on the tepid evidence of ‘Death In The Slow Lane’ at least – be similarly in doubt.

Airs at 8pm on Wednesday 23rd March 2011 on ITV1.