Roxy,

have you tried Craft Pro Zero Extreme? It's thin, stretchy, comfortable and wicks like nobody's business. The XL should fit you.

TE is sold out of the 'regular' long sleeve ones in XL, but they still have:

Zero Extreme Moving Wing Base Layer
http://www.teamestrogen.com/prodCR_1900244.html

Pro Zero Extreme Shortsleeve Base Layer
http://www.teamestrogen.com/prodCR_192443.html

I have both of these and I love them. The only thing that might be a down side for you is that the first one (with the wings) has mesh around the wrist, which makes it hard to push up my sleeves all the way up to my elbow. Then again, that might not be an issue for you.

Cari