From the BikeHub legal pages (as I can't track down my piece which looked into this in more detail - but effectively the principle is here:
"The new Highway Code has cleared up the uncertainty. Old rule 129 has been replaced by new rule 151, which has a new bullet-point on the end:
151 In slow-moving traffic. You should
• […]
• be aware of cyclists and motorcyclists who may be passing on either side
Cyclists and motorcyclists overtaking slow-moving traffic on either the left or the right can now say that this is sanctioned by the Highway Code, as it alerts drivers to both possibilities."
(actually the civil position on motorcycles is now that, primarily, if a motorcycle is filtering, and a car then turns across that bike, the driver of the car is responsible - only in stationary traffic).