£400 seems cheap for a head and gasket repair, the gasket and bolt set is around £100 alone .
I am now on my second rebuild on the wifes freelander, first time kept losing water thought it was the dreaded gasket, turns out the water pump was leaking when under load,still now i know how it all fits together.
Which helped as the new cam belt snapped and lunched the engine the other week.Had the head skimmed and rebuilt just to be be on the safe side (that costs £355).
Suggestions include pressure testing the engine,seeing if theres white deposit under the oil cap,looking bubbling in the expansion tank when its running under load,chemical test (thing like a turkey baster turns blue?), looking for water in the sump,whipping out a plug and seeing what colour it is (plugs ar expensive so be careful).