What you have to be very wary of is what is classed as 'filtering' and what is 'over-taking'. On a recent IAM ride, I proceeded with filtering, but let the Tiger creep to 20 mph. In the de-breif, my observer pointed out that despite the care and observation, this could be frowned on by the law, as at that speed it would be classed as over taking. Also, and a valid point despite what we think of car drivers (but how many of us are that too ??) passing a frustrated driver in traffic, too closely and too fast can easily p**s them off, and potentially make any situation worse.
The guidance I was given was do it, but at a veryu reasonable pace, and be awaer of what affect your actions may have on other drivers, even if they do something daft (how many of you haven't at some time made a mistake), don't antagonise the situation, just be prepared for it, deal with it and move on safely.
Another point raised in an earlier post about the reversing car... Plan yo0ur ride so you don't put yourself in this situation, if you have to pull up behind another vehicle, position youself to one side or another with room to manouvre should the vehicle start backing up, by doing this you are also giving yourself room to manouvre around said vehicle should it stall, or the oppotunity arises to continue to filter.
Also, remember what is legal and what is not.. You can filter past a vehicle at a crossing IF they are stationary for the sole reason that traffic is stopping them from moving off, you MUST not pass the lead vehicle at a pedestrain crossing if they are either moveing or they have stopped for a pedestrian or because signals have ordered that they stop. At normal traffic control lights there is no such restriction, but for safety reasons, it is better to stop behind the lead vehicle as advised, and then if the way is clear and safe, filter past once clear of the lights. Very often you will find cars pulling off at the amber lights rather than green any way, and if your sticking your nose in front, and wait for the green light (as I'm sure we all do), you may well find yourself 'tagged' by the lead car, but if you are positioned behind, by the time the lead car has moved off and you cross the line, the lights will be green, relaxed and in a good place to overtake/filter once the junction has been cleared.