The fault came back 2 weeks ago (shortly after I fixed my starting issue

if you saw the
thread). Last night I had time to strip the V157 again. There was only a little oil in it this time, only really enough to wet the outside of the motor and coat the inside of the casting in a few places and I wiped out what I could.
I suspect that the first time, the cleaning of the motor brushes and commutator didn't work very well because I used meths instead of a proper oil solvent like paraffin.
This time I swilled the motor out in paraffin instead, dunking it up and down a few hundred times and letting it fill and empty in a jar full. I turned the motor a good deal whilst immersed. The paraffin quickly went black, so I think the cleaning has been much more effective.
After re-assembling and clearing the fault code, all is well again. Hopefully this time it will last more than 16 months before needing to be done again.
Matt