So, after reading on the Denford forum that the leadscrew nuts need cleaning every now and then I spent almost all of yesterday cleaning the nuts (without removing them from the lead screws). They were pretty well packed solid with wood/mdf dust and after going through several cans of various degreasers, WD40, PCB cleaner, brake cleaner etc plus a bunch of wooded tooth picks I think I got them prettty clean. A liberal spraying of silcone lubrication and the weird noise had mostly gone so I left the router traversing X,Y and Z for a couple of hours.
Turned on the machine this morning and all was sweet jogging along to the soft limits several times so set up the datums for the work and set ti cutting. First piece finished n problem but after G55 the position had been lost in X by about 3mm.
Swapped out the BOB and, there is no other way to describe it, but jogging in all axis the machine sounded sweeter, sort of less strained. Anyway recut the second part followed by the next operation (about an hour each part) with no problem. Have left the machine on the next operation which should finish about 8am in the morning so hopefully all will be well.
So while the gumed up nuts weren't helping changing the BoB appears to haev been the solution, it may have been a lose wire or a faulty board, don't care as long as it is sorted. Will see in the morning.