when i did bikesafe at castle combe recently,the police had a filtering presentation which was very interesting(included filtering/undertaking on motorways)..filtering is in the eyes of the law perfectly legal past stopped/slow moving traffic providing it is safe to do so but you still need to be alert for other traffic making unorthodox manouvres..once traffic gets moving the onus is on the rider filtering to judge when it is no longer safe to continue filtering(general consensus seemed to be up to about 20mph)..what was more interesting was m/way filtering.again filtering between lanes of stopped/slow moving traffic was fine but best not to use hard shoulder due to risk of debris,also it was deemed perfectly acceptable to undertake providing it was within the speed limit,was safe to do and the rider/driver did not chop and change lanes as this constitutes being unsafe/dangerous..videos were shown of what was deemed acceptable filtering/undertaking..it was a very interesting presentation for most attendees,myself included..
