The heater has slowly started to work less and less. At first it would stop working in traffic jams. Now it's nearly all the time. I have also recently notice that my radiator fan now seems to be jammed on. I assume these are related in some way!!??
Suggest you speak to Julian of JMA Cars, I bought my 75 from him and he knows everything about them. He is especially good with modifications and common faults on 75's, he even sells a fan mod kit which sounds like could be your problem. He sells them online here: www.jma-cars.co.uk/jma/Accessories.aspx
His phone number is also on site, ring for advice first. Mark.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.