The cylinder head is aluminium and while it's lighter than the cast iron cylinder block, it bends easier.
When your car overheated it more than likely warped the cylinder head (even by a . of a millimetre that you wouldn't see with the eye.)
After you have the cylinder head off after overheating, its a good idea to take it to be skimmed (put it on a machine and take a fraction of a mm) off the bottom of it. You need to be careful though - you can normally only do this once or twice 'cos it changes all the compression ratios and if your not careful, or skim it too many times it will just blow the gasket straight away when reassembled
Have a look in the yellow pages for 'Engine Reconditioning' and ring up a few places for quotes - its usually £20 - £30