So a new battery arrived today and has resolved all the problems, although I will still replace the solenoid when that arrives. Looks like the battery was forcing down the charging voltage because that is now fine, above 14v when running. Thanks everyone for all the helpful info and suggestions. I got an Atom AGM battery which claims to be 15Ah and 300 CCA and a 5 year warranty.
I did have to use a ratchet strap to hold the battery in the tray because it doesn’t have the ridge on the top that the standard battery clamp fits in
