We just fitted the weber and it idle great but won't rev. Soon as you rev it nothing happens, you can hear the air sucking but no fuel. However it rev great soon as you put the choke on.

Now, could it be the fuel pump. When we tested it the fuel didn't even squirt out it just sort of ran out.

Could this be the problem or does it sound like something else.

Also when we filled the float chamber nearly to the top it reved great for about 20sec until something couldn't keep the fuel up.

Definitely. If the fuel pump doesn't actually pump fuel, but it only trickles, you aren't going to get enough fuel to supply an engine under load.

On the other hand, it also sounds like the accelerator pump in the carby is not working. With engine OFF, open the throttle by hand and verify that a continuous stream (not an intermittent trickle) sprays down the throat.

You got a worn pump in your carby. Same thing happened to me when I swap carby.
Good luck!