Battery drain (or alternator or electrics?) on Citroen Berlingo 2008?

Car died on M25 after stopping for accident to be cleared. I then have replaced the battery at Halfords as they said it was the problem. Then few days later battery was draining/not starting again so spent hundreds on other 'problems' at a garage I usually trust. After a few weeks with no noticeable problems, noticed sluggish when starting egnition for a couple of days then broke down again ... Battery now draining/dying again ... Is it a 'gremlin' that could cost me and still not be fixed as garage can't promise to find cause? Please help, thanks xx