Its not the bolt that stops the wheel slipping, if you look closely at the picture you'll see a screw missing from the drop-out, bottom left of the wheel nut.
I see the frame is not the only thing with a screw loose...
...JK!
No problem, I know what is missing Paul - you are after the M4 x 40mm bolts which are not screws.
These bolts are 4mm in diameter hence the name M4 and as with any frame if the rear wheel is not on the droupouts tight enough it will pull forwards and in my case bend these M4 bolts used for wheel alignment. I sent you a PM with the link to the bolts.
I see the frame is not the only thing with a screw loose...
...JK!
No problem, I know what is missing Paul - you are after the M4 x 40mm bolts which are not screws.
These bolts are 4mm in diameter hence the name M4 and as with any frame if the rear wheel is not on the droupouts tight enough it will pull forwards and in my case bend these M4 bolts used for wheel alignment. I sent you a PM with the link to the bolts.
Good luck
Tony