When I drive away in a hurry, I hear a clunk coming from the RHS front of the car. Then if it has clunked backwards, it clunks again when I brake hard.

Without fail it will do it, but only when driving which means its impossible to find unless I site on the engine when driving which didn't end well last time.

Anyone know what it might be? Started after my timing belt so I guess I'll recheck all that first.

i'mguessing that one of your engine mount were possibly left loose.

load the car up in forward and reverse (select the gears and accelerate with the brakes on) while you have your bonnet open and get someone to look and see if the engine moves much.

Check suspension components too in similar manner.
I do know a story about a guy who was driving with 2/5 wheel nuts with similar symptoms tho...

I doubt it's CV cos it rattle more constantly.