Volvo C30 battery drain

First thing to do would be carrying out a load test on battery to eliminate any weak / dead cells . If o.k you need to carry out a parasitic drain test

Thanks for getting back - battery and alternator brand new - ran an alt/starter/battery start/stop tester which came back with excessive ripple - does this mean that I have a knackered diode on a new alternator?

Never heard of that term before " with excessive ripple " I would normally carry out a LOAD test on alternator and check voltage / charging rate . If it was a reconditioned / replacement alternator there are quite a few that , how can I say it politely , leave much to be desired