Almost certainly a head gasket, what you are seeing on the filler cap is what is dubbed 'mayonaise', this is from the coolant and oil mixing, the bubbling is because the cooling system is bein pressurised by the engine.

The real solution is to obviously get the HG replaced, for a garage to do this it will be expensive, you could also try the likes of a product like "K-seal" this is added to the coolant and it can be a cheap workaround.

Yours is a 1.8 (1753 CC) engine, this is an old Escort engine.

On another note if the oil is black this may want changing, although on a Derv it will get mucky pretty quickly, it's worth doing if you don't know the history!

Martin