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If I leave my petrol tap on the carb on my VBA 150 floods from the top of the chamber where the fuel filter sits. It is a little lumpy on tickover and light throttle which I presume is a result of the carb flooding..

Am I right in thinking that the needle valve should cut the fuel off completly and it should not flood even with the fuel tap left on?

I've tried another needle valve from another carb and it still floods (though that needle may also be damaged).

Does it sould like the needle valve seat is the problem?

Bryn,

Only 2 possible cause's for flooding which is Float sticking in chamber or as you have said, a worn needle. If needle valve seat is worn then there's not a lot I think that can be done

Tru, though the fuel is coming out the top chamber above the filer, which is before the valve (?) so maybe its just the top seal thats duff and the valve is actually working fine...

Check your float, it may not be floating, I had this with my sprint, there is an overflow hole in the lid above the filter, if the float doesn't float, petrol keeps on flowing. I suggest replacing both float and needle valve.
Good Luck
Tony VCB 3127