'Engine system fault'

[Lozbaxter]

I need some help!! I am having what I thought was a battery issue with my Ford Focus 1.6 Titanium. I left it overnight in a station car park (30 hours in total) and when I arrived back the battery was dead. The key turned over, the lights came on the dashboard, but the engine wouldn't start. I was able to get jump started and drove the car home (20 min drive). The next morning the battery was dead again, same thing, lights came on, key turned over, but no engine start. Later that day after work, my husband tried and absolutely nothing. No lights, nothing, even the locking system went down, and an 'engine system fault' error came on the display. So it was like the battery was so drained it couldn't do anything (we have searched the car and there was nothing obvious left on that should have drained the battery). We bought a new battery which my husband fitted last night but which has not resolved the problem. The lights come on on the dashboard, and the locking system now locks again (so battery issue seems to have been fixed) but still no start and the 'engine system fault' error is still showing.  

Can anyone please advise what could be wrong and how we can get going again? It definitely feels as though the battery was the problem (at least initially). The car is on my drive at home so I can't even get it anywhere to have it looked at (no breakdown cover so trying to avoid a call out as much as possible!!!).

Please help!!

[pragmatix]

I would start by getting the battery fully charged with a battery charger, new batteries arnt normally at full capacity.