Paul, Not quite as good as that. I've a piece of 3" x 3" wood running inside the shed, screwed into the main frame with 6" screws. The daugher's bike is locked to that with two decent locks and mine is now locked to hers with a Trelock with an armoured cable through all the wheels. I hope that does the job. The blighters should now have to demolish the shed to get the bikes out. As I said, it was just about the only evening in the past five years that they could have nicked both bikes.
Thanks for the suggestion AS. After careful consideration of your suggestion - which was most appreciated - I have decided that "a brain-damaged pit bull terrier" is perhaps not the best solution for me given my personal circumstances. Does that deserve a smiley? I think it does, so just for you