Yes it is the battery,
12v 6.0 Ah from somewhere like screwfix, maplins, B&Q,
Yes you can fit yourself, just remove the fuse from the fused spur, or isolate it at your consumer unit before you remove the panels cover.
Don't open with mains power present!,
Remember safety first DIY second.
Good luck, be safe :)
e manno
September 2012