Russ If it's the EGR cooler then a simple test of the N2O levels in the exhaust should show it up. I doubt the head is porous,these days they're built like tanks,it may however be warped and an honest garage would skim the head. Talking of honest garages (they do exist!),it's a long time since I used them,but Morgans Garage in Pield Heath Lane Hillingdon (300 yards before the hospital coming from the Uxbridge Road) used to be excellent. Phone 01895 254312. I live in Scotland now and am a bit out of touch with services down there. I've found a great garage up here,so I suppose you could blast up the M6,that would probably identify the problem ;0)Good Luck with it.
Tony Wood ● 5504d