Thanks for that, Ed. I now have a few places to try if I need them, although it seems that Spitfire Mk 4 had 150 lb. springs anyway, which is what Mister Towed recommends, although he has a 6 in his car.
I was recommended also to Faulkner Springs last night, who have a very easy to use website.
http://www.dfaulknersprings.com/
Cheers,
Martin