I thought I had better own up to my stupidity now so I can have a clear(ish) conscience for the afternoon.
After fitting a new chain it's a quite a good idea to tighten the wheel bolts properly.
Goinig to work this morning I wondered why my bike seemed to be a little crooked, perhaps last nights collision had bent something, but a quick looked couldn't find anything amiss, apart from the rear wheel not being completlely centered. Oh, I must have been a bit careless when I changed the chain and not centered it carefully. I'll sort it out tonight in the dry at home.
A bit further on accelerating a little going up a slight slope there was click. Hmm, that's not good. I stopped at the top and saw that the back wheel had moved far enough over to touch the rear forks.
So I stopped to have a closer look and saw that the chaintug had slipped - it was pretty obvious why, I hadn't tightened the bolt, nor the one on the other side.
Whoops. It was last Tuesday evening that I changed the chain, so either I was forgetful and forgot to tighten the bolts or someone had tried to nick the wheel and got as far as slackening the bolts before noticing how it was locked.
My money is on my stupidity!