If you had the head gasket replaced without renewing the coolant then it could still be contaminated with oil.
You should renew the coolant, you could do it yourself for just the price of the anti freeze around £20.
Always helps if you don't keep your car details separate, most modern cars do not have a drain to let the coolant out so you would need to remove the lower radiator hose, drain it then flush clean water through the system for a few minutes, re secure the bottom hose then top up with new coolant.
There are usually at least two vents somewhere in the cooling system you need to open to get rid of air locks which can cause problems if not done correctly so it is a decent price for a garage to do the work.