Thanks for all the suggestions!
I thought I needed a FAA medical regardless of what licence I was aiming for in order to fly solo as a student in USA airspace. (I already have my JAA medical). Can anyone confirm if this is the case? I was thinking of getting it done in the next few months so it's one more thing out of the way.
I reckon I will need a minimum of 15 hours as the schools I have contacted all want 5 hours dual to refresh, and then I need 10 hours solo. But I would probably aim for 20 hours at least because it's so long since I've flown I think I may need more than the 5 hours minimum. So if I can tie that in with a FAA licence I will do it I think. My only worry would be if time constraints meant I would be risking getting my JAA licence, as that really is my priority.