Well that was an expensive lesson learned by all.
Popped my head out the front door to deposit a spider humanely outside, when ds2 decided it would be hilarious to push the door closed behind me. It's one of those self-locking front doors - worse, with the key in the lock on the other side, I couldn't just use a spare key to open up.
NomDePlume was a star, popping straight over (i had my mobile in my pocket fortunately) and when her coat hanger didn't result in an open door, went round to find a neighbour who was in who also gamely tried to open the door while we tried to calm a hysterical little boy through the letterbox and window.
dh came home, couldn't open it either - had now been about 40 minutes so had to call the locksmith. £100 and ten minutes later ds2 and I were reunited and he's not showing any signs of letting me go in the near future.
Moral of the story: always squish spiders, don't try and be humane.