Loss of compression

Hi Guys, I'm having a right day of disasters with my defender, she has a 200tdi disco conversion, I have compression tested engine and on 3 pots I am get 420 but on pot number3 I am only getting 350. Is this a problem? I am changing head gasket due to oil leaking from it near pot 1, will this solve the problem or is there nothing to worry about. Any advise more than welcome please.