Thanks for answering Gents.
I'm absorbing your suggestions. I like the idea of both the TR7 and the Volvo.
I do like the look of these though; https://www.youtube.com/watch?v=XVBiDLUo_1Q
If one could be modified to fit the Sprint, I'd be very happy, but I don't want to spend $270 to find out that it wont fit.
A mate has one fitted to his Stag, its build quality is first class, he loves it. Apparently Stags had similar issues with their factory header/expansion tanks....is that right?
That tank won't fit a Sprint, although I dare say you could cut one up to make it smaller?
Also flitch panel positioning is unsuitable ideally because it is too low
(although it is true that some manufacturers do employ such arrangement but these have bleedscrews
fitted to hoses to try and avoid air locks (Renault 19)).
I have never studied the Stag but don't these have a radiator with a cap?
(I am pretty sure the Stag engined 2000 saloon prototype belonging to Canley Classics
(which I saw at Ingliston many years ago) had that?)
The car I looked at for ideas was the Saab 900 having been inspired to do so by Jon Jackson's use of a VW header tank on his Sprint.
(The Volvo tank has a better shape for fitment to a Sprint than the VW type and a bigger capacity too.)
My preference is for a screw fitting cap.