Hi Paul, if there is no visible signs of an external leak then it is most likely that the head gasket has gone or worse a crack in either the head or the block, have a look at the oil filler cap if there is a "salad cream" deposits on the cap then its an internal leak!! you could take it to a garage and ask them to to do a "sniff test" they will put some blue liquid into a tester and sniff the air in the expansion tank if the liquid changes from blue to yellow the combustion gasses are getting into the water channels and this will confirm the head gasket or worse
nigel
May 2008