Be good to do a compression test.

Could be head gasket on way out.

Admitadley not the cheapest job in the world.

Compression test should show major difference in pressure in each bore.

Get a kit from halfords for about 15

quid or go down to your local garage and get them to do a compression and leak down test for you.

Shouldnt cost much as wont take long.

Are you getting through coolant? Is your temp needle doing anything abnormal? Running high etc?