[Big Dave]

It all got very messy around that era of what the recommended cambelt intervals were.

Here's an old service laminate for the Astra-G and all the different change dates/mileages.

Might be worth a quick call to a Vauxhall dealer to see if there is any updated info.

IIRC, the 8v engine is non interference engine. It's not usually the belt that primarily fails, it's the waterpump or tensioner instead.