spare parts required?

i have a 20 year old warm air heating system made by thorn emi model 25/30wh which has just broken down for the first time in the 15 years since i have lived there, the engineer has managed to get it going again (it seems that one of the "thermistor's" is breaking down) unfortunatly he tells me that spares are very difficult to get.
Does any one know where i can get part No 212A303 for the above model or a break-down of components in this assembly as the "thermistor" thats playing-up, if one is available would cost around £5.00 but its knowing the correct type and temperature range, alternatively he can fit a new heating system for around 5K
I know which one i would like to go for!!!!
THX