That's good news Sue, quite an honour to be contacted by a gallery, in my experience it's usually me doing the asking!!
From the the ones I've dealt with and the research I've done online, seems like most galleries have pretty standard T&C which they should be able to provide you with right away. It seems that most often they'll take work for a period of time on a "sale or return" basis ie you still own the artwork and if it doesn't sell after a mutually agreed period you get it back.
If it sells they'll take a commission, the ones I've dealt with take between 30% and 45% but I believe some galleries charge up to 60% (maybe that's in the bigger cities, don't know). The commission works on the selling price so make sure you do your sums right! eg if you want to end up with £150 and it's a 40% commission the selling price needs to be £250.
I've always been very hazy on how VAT works, I'm not registered (well below the threshhold har har

), if the gallery is VAT registered just make sure that's all taken care of in their commission. Insurance seems to vary, most of the ones I've dealt with insure the paintings while they're on exhibition but that's not universal so you should ask.
When you leave your work you're supposed to get a written receipt to prove what they've got and also just in case there is a problem with the gallery it establishes that the paintings are owned by you not them.
I bought a book a while back "The Artist's Guide to Selling Work" which I found quite helpful just to give you a feel for what's normal. It's by Annabelle Ruston, costs £9.99.
I'm sure other people have other things to add but hope that helps for now!
cheers
Elise