David_Dickson

Get triangular wedge of wood and pull out the top of the door frame. As you pull on the frame, push the wedge in to hold it open, then use stiff wire or copper pipe etc to hook the keys. It takes seconds and causes no damage at all - You are not some joyriding chav who would hoik the frame wide enough to get his arm through and ruin the door, it only needs to be opened by 2 inches, and there is plenty of "give" to allow this.

Dedrater

c_maguire

As has been said you can lever/wedge the door out at the top and then use one of the 'grab' tools (bendy thing about 2ft long where one end is like those grabbers at the fair where you win a rubbish toy if you can pick it up and get it to the chute) to lift up the lock on the door. 306 should have an old style up/down lock on the door.
If not then use something to hook up the keys as David said or push the central locking button to unlock the car.

I expect you've probably done something by now though.