What engine? Blue smoke indicates its burning the oil, on the zetec's it's common for piston rings and valve stem seals to go, as it''s when you boot it, I'd reckon on rings. Smoking on over-run would indicate stem seals. £715 is a lot, I'd say stick a second hand lump in for a few hundred. Also, if it's a zetec se engine, they're not supposed to be rebuilt iirc, so I wouldn't bother.