Hi Ed
I appreciate the time.
In answer to your questions,
It's never had a wall box in the 2 years I have had it (I take it you mean the item that plugs onto the "ST5" plug on the bottom right of the PCB). I tried to clear the errors by doing the P033 6 then H but this didn't work, assuming I did it correctly.
My next step will be to replace the battery as you suggest and see if this clears it.
Cheers for your help.
Scott