The bolt came out, it was just a bolt through into a nut the other side, I managed to shear the bolt at the start of the thread so it slipped out quite easily! Will upload some pics shortly
Ah yeah, I forgot it was quite an old frame - I was thinking of mine, which is just threaded at the top of the stay, rather than having room for a two-part fixing. Maybe go to a bike shop and see if you can get a proper seatpost bolt - might look nicer?
I don't understand how you've even managed to bodge a frame with an integrated seatpost bolt to take a QR, but I like the initiative - pics?