hi mate i have had this problem myself and in the end i found it to be 2 injectors stuck open so it might pay to get these checked also try water temp sender
I HAD A TOYOTA LANDCRUISER AND FILLED THAT UP WITH PETROL INSTED OF DEISEL DRAINED IT FILLED IT UP AGAIN WITH DEISEL THIS TIME AND IT WAS FINE YOUR MATES SHOULD BE TOO
in your case i would take head off buy a glaze buster from halfords or some where £20,and deglaze the bores, fit new piston rings and change valve stem oil seals as they are proberbly rotten new head gasket and bobs your watsit