Just a little update to this thread from while ago.
This Roomba was working intermittently, It would basically work fine for a few days and then would either just switch off whilst charging, shudder around whilst cleaning before coming to a stop and switching itself off and then wouldnt turn back on for a few days, or most commonly it would seem to be fine on the charging station but when you pressed clean it would make all th eusual sounds and movements but at half speed (beeeeeeeeeeeeeeeeep beeeeeeeeeeeeeeeeeep...........etc) and would eventually just shut off.
After lots of messing and testing and using a lot of google searches I found that the piezo crystal was at fault (small silver canister on the main board with 2 legs). Being an auto electrician I am pretty good with a soldering iron so replaced the offending article with another one from a burnt out 530 pcb and hey presto!
The Roomba has been working fine for severall months with no issues at all.
I thought I would share this info just in case somebody else is out there with the same problem. I know I could have just bought a new pcb but they are around £50 on eBay
whereas a new crystal would have been about £3.