Hi Dominic,

Thanks for your response. My car is at the garage and they discovered the fault, I haven't actually seen the part and I don't know exactly how it looks. From the way they describe it though it sounds like an air-oil cooler rather than a water cooled one. Apparently mine has a crack or pinhole somewhere which is weeping ATF, meaning dirt is sticking to it and filling the cooling fins.

I know Rimmers show that diesel transmission cooler as the correct part but that doesn't fit with the garage's description. I'll ask them to send me a photo, it's a three hour round trip otherwise I'd go myself for a better look.