Nick,
Seems the chassis is in good nick, though there are slight variations with the one I photographed.
Most chassis I have seen were badly rusted, most of them beyond repair really. I'd say a good buy at £53.99 + transport!
Those wheels are fitted to a variety of British vehicles. Note the come in three, four, and five lug nut versions. Your mortar trailer should be fitted with six lug nut drums and wheels.
Good luck,