Did you check the remote control batteries?
This sounds to me like a behaviour when batteries are low.
First thing I did was change batteries, although it worked fine the previous day on 6.1
Anyway, just came on to say I have resolved it thanks. It appears as though one of the buttons on the front of the STB had been stuck in slightly which explains why the volume went straight to 0 by itself!
Must have been a combination of muck and it someone putting something in front of the STB.
After going back to 6.1 then 4 I knew it couldn't be s/w related, but I'm back on 6.2 and happy again now