Yay, problem solved, yes!
My little dip pipe mod seems to have done the trick. Spent much of today hammering round North Wales and not a sniff of air has made it to the radiator. Weather was cloudy with some sun but great driving conditions.
You know it's amazing how friendly people are when you have an interesting car - coming over to chat and so on. I was getting petrol this afternoon (this is required quite often) and the whole crew form the tyre place came over for a look. The boss, who knew his stuff was talking to me about the suspension because I mentioned I was trying out more rear toe in, so he offered a free tracking check once I'd filled up
I'm now running 3mm toe-in at the rear and it's amazing how much more planted it feels. I have also got the diffuser fitted now which might make a bit of difference too I suppose, but I'm inclined to think it's mostly the geometry!
Richard