Thanks Jazz will try what you said. I should point out that this is a new machine only used about 3 evenings so there shouldn't be anything worn. I have tried to search for problems with this machine and found some Youtube footage of a similar problem and they said it was the wires. I have tried jiggling the wires to reproduce the fault but can't. Also the Y axis wires never move.

I'm a total novice to CNC, is the lead nut the nyloc that is on the end of the y axis leadscrew? I contacted the seller (ebay) and they said not to touch this nut. Actually they seemed really helpful and quick to respond, but since I sent them the video they are ignoring me. I wonder if I have gone outside of some Paypal claim window or something, and they were trying to fob me off until that time.