Mystery solved

The mystery of the rear lamp failure has been solved (although not tested). It appears that the Lamp Outage Module (LOM) monitors all the rear lights in one go, including tail lights, brake lights, reversing lights and number plate lights. Turns out that the high level brake light has 3 bulbs in it and although 2 were working the third had blown. This is why the LOM was saying that a lamp was out. I'll get a new lamp today and all should be happy with the beast.