de dietrich cx4844u1 range not getting to temp

cust rings up and complains oven isnt getting hot enough, but it is getting warm… wants to know what they should fit, we suggest it sounds like a thermostat as its not getting to temp, so were asked to quote for the part, qtx is 60ish+vat to us, so we quote accordingly so cust goes away to discuss with hubby. hubby decides were too expensive, and finds part online for £60 inc vat and p&p. anyways, we get a phone call, hubby cant work out how to get into range cooker, could we tell them how to get into it. HA no way, not least because weve never taken a DD range apart, so they ask how much to come and fit it (Std callout, £48+vat). so we go do the job, and its not seemed to make the slightest difference. we go away saying we’ll look into it, and didnt charge them for the fit *doh*. Since then get a call later on from hubby refusing to pay us because he thinks we havent fitted it right… and we told him it was the thermostat and hes bought it bla bla so now bit at stalemate. dont really want the aggro of court etc, and not fancying writing the time off, as shes the pa of a landlord we do a lot of work for, and she just wants the oven to work, anyone got any ideas?

symptoms:

m/o heats to 180* according to t/stat+neon, (within 5 mins) but our cooker thermometer is reading consistant 120-125* for about 15 minutes on fan oven function. on fan assisted conventional it gets up to 135 for same 180* setting. but doesnt get any better.
temps all taken on middle shelf.

am wondering a: duff stat – its different to the original, b: hot spot around the phial, so its recording the right temp in that location, but rest of the oven is cooler, or the fan isnt pushing the air hard enough around to circulate properly, but theres no serious labouring noises, or even wear noise from the fan (that sort of wow/flutter sound) so bit at a loss. are there any toc’s that will affect m/o temps by cutting out the element but still allowing the t/stat neon to operate on/off normally, and would the cooling fan listing slightly and causing a tinkle noice affect the toc’s cutting in early??,