Thanks for the new thread OP. Apart from my suggestions about a locksmith and a hula hoop, I’m at a loss but will continue to give it some thought.
The fire brigade or reporting a gas leak, as PP have mentioned, might be worth pursuing.
If you’re not keen on those options, is it possible to lift the catch of the window you used? Assuming you manage this, is there a larger window below it which could be opened to release him?
Fingers crossed that the new owners appear today.
I hope you managed to get some sleep and that you’re reunited today.