Took the newly built bike out for it's 2nd ride tonight, and I now have a snapped mech hanger, several bent spokes, and no idea what caused it.
I was pedalling away, not going mad, on the big ring in about 5th / 6th at the back.
I don't know if the chain jumped off, dragged the mech into the rear wheel and snapped the hanger or what, but I'm thinking it couldn't because even if it lost tension, it would surely move away.
Anyway, I now need to get the wheel into LBS and hope it's repairable, buy a new mech hanger and possibly a replacement rear mech, as it's twisted.
On spinning the chainset, the big ring does look slightly bent, and it was the first time I'd used the big ring since building it.
I feel the need to hit something with a hammer now!!!