I may be being paranoid...

Abus chain lock - probably worth more than the bike.

Will just keep an eye I think, but action will have to be taken at 5:30 if it's still there. Thing is how am I gonna cut a fairly befty lock?

Sounds dodgy to me, using a decent abus on a crap bike.

Can you get hold of an angle grinder and extension lead? If so, cut the lock off and leave a note. I wouldn't go to the trouble of putting another lock on his/her bike, cos that'll only get cut too.