Damaged engine?

[Southwell]

Well i went round my parents for lunch yesterday and my mum just said a comment in passing. 'A light comes on when i go round corners, it's been on an off for a couple of weeks' Ok, so straight away im thinking oil light, which is confirmed after turning over the engine and hearing the tappets make a noise like ive never heard in her car.
The oil was very low, not even registering. So i go to fill it up and low and behold i find some nice mayonnaise on the oil cap and inside the rocker? box. I know this can happen on short journeys, but it seemed quite excessive to me and im thinking head gasket. As far as i know it hasn't overheated yet.
Anyone got any suggestions? The tappet noise did quieten down after the engine was to temp but it was still most definitely there, and i have not heard it on her car before so i am thinking that the engines been starved of oil and it's not a waiting game until detonation.

It's a 1.6 Astra 2001.

Cheers for any advise.