I think my PRAM battery is dead (date resets to January 1st 1970 when started up with main battery removed) but I can't find a replacement PRAM anywhere (I'm in the UK). According to iFixit the part number is 820-1686-A - is this the only PRAM module that will work in my machine? (I've seen lots of other similar looking parts on eBay, but with different part numbers). Also, is it not possible to actually just replace the battery, not the whole circuitboard? It looks like a regular watch battery I could pick up for £5 from the pic.