So I removed the interior and got to the BB for my first time last Friday and found the orange plug about 50% unscrewed. Quite a bit of oil-spill evidence, but it has never dripped onto the floor. Seems it had VERY little in it, despite having supposedly had the oil changed at the last service (500 miles ago). I added approx. 300-350cc of fresh 85W140 Omega 690 gear oil and unsurprisingly the BB runs much quieter than it did before.
I had noticed more noise since the Centa was fitted at the same service, but I put the increased whining down to the fact that the compensator no longer rattled like a bar-steward, so I thought little more about it. However, it is now clear that the BB was very low on oil, so hopefully little damage done.
