So made it there and back.... just.
Only problem was after 4 days at the festival without an EHU, I was being particularly careful with the voltage so no radio and even dimmed the dashboard. As a result I missed the engine temperature going through the roof about 15 or 20 miles from home until I saw water flooding out the back of the van.
Immediately pulled over and turned off the engine, in the knowledge that there wouldn't be enough voltage to re-crank the engine.
Called the RAC and as the engine had cooled, we jumped it and moved the van to a little service lane on the M56. Under the van and identified a clip that had rotted off a pipe carrying coolant to rear heater controls that I didn't know existed. (They have been hidden behind a cupboard during the van conversion.)
Replaced the jubilee clip along with one on the return pipe. Filled up with coolant, jump start and back home in under 30 minutes.
Van now booked into my local garage to check the alternator and also run an eye over the cooling system.
Every trip is a learning experience.