Thanks to all who have posted! Based in the East of England, but we don't mind where we do it! Apart from the 'normal' testing houses, which we have found pretty expensive, Universities seem prety good value and very helpful. A certified lab is not necessarily vital, we'd rather have good service and communications.
Actually what we are struggling to find (at a good price) is the ASTM G53 (or superseeded version) UV B testing, this seems a bit rarer.
Oh, PS - It has to be B117 - I'm not a fan of the test but that's the spec' we have to match unfortunately, there are much better 'realistic' tests around I know (grumble)...