Originally Posted by stut
If you do change at Clapham Junction, then head for platform 10. It's a confusing station: there are actually 5 different platforms serving Waterloo, but 10 is the one to go for! Note that there's quite a few stairs.
Agreed on Platform 10 - even more trainspotter of me (!) - the fast Southern trains from Gatwick arrive on Platform 12, so down the stairs, turn left & back up the next steps on your left to Platform 10! Make sure you get the Southern trains and not Gatwick Express, as Southern trains tickets are cheaper and Gatwick Express also do not stop at Clapham Junction.