Sounds suspiciously like you have a failing battery. Lots of info on here once you get your questionnaire back to Tim and become a full member.
Nich.
Yes Nich, I would say 100% certain. If you recall my 'Olive' did EXACTLY the same. Your advice then was invaluable (and accurate).
All cured simply by fitting a new Varta in place of an 18 month(!) old Bosch.