I bought a Hatsan (Escort semi-auto) 12 bore for exactly the same reason and have found it to be very reliable and not too fussy on ammo. I bought the camo version and have found it to be very robust and it is still in excellent condition despite a lot of use for wild fowling, pigeon shooting and the like. I would recommend buying new which would still leave a lot of change from the £700 budget you have.