Thanks for all the replies, and yes, hunting around on forums whilst the production system crashes & burns is never fun to watch.

We got it in the end, it was Tivoli backup process that although was turned off somehow was still being called. It was continually trying to fire and taking memory each time, until crashing SQL.

We have fully removed it from the system and the memory counters are looking like normal again.

Am off for a beer 🙂

Andrew