Oil in the Water?

[Squeaks]

No Not mine. Phew. but my aunties Mk1 ZS180

i was speaking to my uncle last night & as he don't have a computer I'm asking on his behalf.

he was checking all the levels on the Z Saturday & noticed that the water tank ( expansion tank? ) had oil in it, it was floating on top of the antifreeze & didn't go right down to the bottom of the bottle.he took it out & checked the dipstick & it had no mayo on it. we seem to think its leaking it in from the head gasket, how likely is this to blow? i know with the MGF when it went it put water in the oil. but this is the other way round, is it just more of a annoyance or worse?

any help would be appreciated & ill pass the info onto him, my auntie is so worried as she cant afford it to be major.

Thanks.