So I jump started and it fired first time. All good although at that point wasn't able to measure voltages.
Went on good long run keeping revs up, no lights/wipers to drain power.
Left for 24 hours with battery disconnected. Now able to measure voltages (drove home to pick voltmeter and stuff up). Battery has 11.8v, which drops to 11.4v when turned on (pump presumably). Engine won't turn over, starter just clicks (series of quick clicks).
Battery now on charge (CTEK).
Unable to confirm atm that there is a secondary earth strap to engine mount but earths that I have found are clean and secure and no difference between chasis and block voltage readings.
My plan is to charge battery, start car, run and take voltage readings.
Happy to hear what others would do in these circumstances. (yes, phone call to CC on Monday if not resolved is on my list). TIA.