Heading back to my Mother's house where the XM lives and thinking about why the alternator light was on, suddenly noticed that the ABS light was on too. Much swearing ensues.
Got just past Beckhampton roundabout and power starts to fade noticeably, a few seconds later we are coasting into Silbury Hill carpark and come to a full stop with a dead car.
45 minutes later the AA man arrives and confirms dead regulator on the alternator.
Pops in a fresh battery and the journey final ends Mother's garage.
Nice AA man pleased to work the the XM

Anyway the battery is now in Calne connected to the charger. The alternator is now in pieces. This has confirmed that the unit is pretty new as the brushes look great and the commutator is also unworn.
The unit has an unreadable label but is stamped Valeo A14N139.
I am struggling to find a suitable replacement brush/regulator unit on flea bay. All suggestions welcomed.
Rob