Well I took the tank off, disconnected everything, left the two hoses off from the drains under the tank and put it back on and it fired into life first stab of the starter and revved cleanly. So, im none the wiser exactly why but it seems ok! Cant take it out as mrs V is out and the kids in bed so hopefully get a chance to run it round the block im the next day or two and see if its stable now. Fingers crossed! Almost annoyed I dont know what caused it, but grateful at the same time