Are you sure it's definately the head gasket?
Oil coolers can fail and show similar characteristics. Oil in the water in the rad., sort of scummy deposit on the water in the header tank, but no water contamination of the oil (evidence of emulsified oil in oil filler cap) generally means oil cooler failed. Because the oil operates at a higher pressure than the water, oil leaks into the water but not the other way.
Does it overheat badly, this shows gasket problem.
Oil coolers are relatively cheap (less than £50). If it is the gasket and you do it yourself (not dead easy unless you've done quite a bit of spanner work) make sure you buy new head bolts and be scrupilously careful you torque them down exactly as it says in the manual or it'll probably go again. Good luck.