I had another look at my 'bite this evening and made an important discovery that has given me great hope of getting it working properly once more. I was going to re-flow the solder on the bumper switches but when I got the mobo out it looked like there was nothing wrong with the solder joints at all.
So I checked the operation of the switches with a meter and sure enough they were not working at all- either one! I checked between all the combinations of pairs of the 4 of the soldered contacts and none of them showed any change at all when the switch was depressed!
So I took the switches apart
The disk to the right is the domed contact that clicks when you press the button and is very badly tarnished- no wonder the switch didn't work! The other one is the same. Needless to say I couldn't get the switches to work properly again so I soldered shorting wires across the legs on the PCB.
On reassembly my trilobite now seems to be working much better- it now starts reliably- with the ultrasonic clicking and the fan spins up and the brush rotates, but now I only get a "check wheels" error #6, but I was expecting that.
So the bumper error AND the error 4 have both gone- just need to fix or find a replacement for the wheel/gearbox and hopefully my new Trilobite will live again. Oh and I'll have to figure a way to fix the switches as I think the bumper function is quite an important one!