Mik,
OK, Get the air cleaners off so that you can see into the carburetter throats. Hold the air pistons up. (you'll know 'em when you see 'em) while somebody turns the ignition on. DON"T HAVE THEM CRANK THE ENGINE!. You just want the ignition on. This will activate the fuel pump. If you can't find somebody you can trust, go ahead and do it yourself. You'll just have to move with reasonable alacrity not to flood the engine. Watch for fuel to come flooding out of one or both jets (the brass tube in a hole that the long needle from the air piston pokes into). If fuel comes out of one or both, you have a float problem. This could be float valve(s) or sunken float(s). The only other plausible cause is the wrong fuel pump being installed providing too much pressure. If you do have a float problem, you'll need to remove the carburetters. Order gaskets at the least. Let me know.
If fuel doesn't come flooding out, we'll have to take a different tack.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon