The EA regulation for this kind of activity states processing 250l of waste at once as a limit, however we are domestic so unregulated. When I spoke to the local EA guys on the phone, they confirmed that as domestic operations there was no legal requirement for us to be within this limit but they said they would rather we complied with the regulation limits anyway. "Don't do anything to warrant our attention and we'll leave you alone" was the message - although delivered a bit more cheerfully than that (they were friendly enough about the whole thing!)
So I think that yes we can make large batches (legally) but for the sake of staying under the radar it would be reasonable to stay under 250l of waste at a time.
I'm kind of curious how Dick got inspected and had to build bunding for his oil stores as a domestic operation that is supposedly unregulated. But perhaps that ties more into oil storage for CHP.