thanks for your help guys, the actually screw on cap is ok, but on top of this cap is a little yellow cap, which stops the float sensor from coming up too high, it's this yellow cap which has fell off, i think what happened is when my engine was re-mounted, it was not pulled forward enough, which over thecourse of a few days rubbed against the top of the brake cap, you can see it's been rubbed against on the top, causing the yellow cap to fall away i think.
thanks for your kind offer burp, i've sent you a pm.