Filtering in very heavy traffic you're going to end up stuck at some point. I just go down the 'middle of the road'. By which I mean wrong side of the road. The right hand side means you can see where threats are coming from (oncoming traffic/twats) or are so far removed from it (stationary traffic with the gaps full of cyclists) that as long as you can filter back in you're okay. The problems with splitting lanes is you're neither fish nor fowl, so to speak: you may get stuck in traffic which will scream off faster than you and you're annoying people by squeezing past they're vehicles (I know, I know), but if you're splitting you're going to need their help later on.
The trick with filtering isn't to my eye filtering at all but the usual confidence, making space in the road and being able to be in the main flow of traffic without being intimidated. Whipping between cars unsettles people and keeps you hidden: it's great, thrilling fun but the reason we have the adrenalin is it is a touch unsafe donchathink and seems to me a little underhand? Screaming down the wrong side of the road may seem foolhardy but your visibility is great and seems less annoying, or, more importantly, is less likely to annoy people you have to deal with later as they'll be behind you. It is merely overtaking, after all. Watch out for U-turns though.