Thanks for the replies everyone. I am very much a noob at this too. I know what trees look like on the outside pretty well but not inside. Lack of funds means I am exclusively using reclaimed and palletwood. It's surprising what a wide variety of woods you can find, and more hardwood than I expected.
BM101, that's a great Thor's hammer! Can't wait till my kids are old enough for this kind of project. You know I said birch, not beech right?
I suspect birch because the bark is white with lateral stripes - which you only find in birch or cherry trees and it's not cherry. Also I read that it's hard to see the rings because the yearly bands are a similar colour. It's pretty heavy, and fairly hard, although it hates being planed against the grain, when it clogs up like a softer wood. At first I thought it was cheap white pine as it's light coloured but it doesn't have many knots and the grain looks wrong.
I've read that in some northern hemispheres birch is the commonest tree so who knows. The pallets do not have the international shipping stamp though, so it may be UK made? They held gravel boards.
I'm using for a Paul Sellers style workbench, it's taking forever to plane square. It's not in great shape at first as you can imagine. I am chuffed at the thought of a solid hardwood workbench though, if birch it be.