Seat post bolt probably won't be M8 - maybe M6 or even M5 - nip into thorns tomorrow with the remains of the bolt and they'll sort you out for a couple of pennies.
Went to the almighty expensive orange and black sign in the sky and bought myself a quick release, not the best, but it's a quick fix, need to get remains of a chain to link to my seat now though! Also they only had silver, contrasts with my mostly black bike! :(
Seat post bolt probably won't be M8 - maybe M6 or even M5 - nip into thorns tomorrow with the remains of the bolt and they'll sort you out for a couple of pennies.