Latest update- the engine oil has all been cleared from the bilge, took the filter off, the gasket appeared to be correctly placed. Put it back in and tightened it up, checked around the sump plug / pump entry and there didn't appear to be any problem there. No oil cooler and no inline tap on the sump pump pipe. Filled up with oil, ran the engine, gave it a few revs and ran it for 10 minutes or so with no further leak.
Hopefully it was the filter - we'll set off soon from Sale to Lymm and keep an eye on the bilge.
Just one other thing that probably isn't relevant, the tachometer isn't working now.
If it happens again we'll call out RCR.
Thanks to all for your input