Already checked that, not "sorted".
I've replaced many MB batteries in my PC days when they didn't know the date and the only one I can find on the mini is this one for the PRAM:
As I said, I lose a couple of minutes per week. If I open the date/time preferences it will refresh but start to lose time again. I even tried changing the ntp server.
I'm going to order the tools and battery and just be done with it, thanks anyway.