Cycling Techniques

to me assertive cycling is riding in the traffic stream either when you can match the speed of the traffic flow (most of the time in london) or if you need to to minimise risk such as when passing parked cars, a side road or riding in a bus lane.

Absolutely true (the second bit); I was just on the phone to Festus talking about this thread and trainees who find primary position such a liberating revelation that you have to remind them that riding in the middle of a narrow side street with parked cars on both sides does not mean they have to ride in the middle of every road from now on. As SkyDancer says, you adapt to the circumstances. I think Ed's pic is of the bus lane near Clapham Common - anyway it's the same - and it's an odd one because the Super Highway bit makes the bus lane far too narrow for a bus and any rider sticking to the cycle lane is going to be dangerously squeezed out if they do not discourage busses from overtaking too close. But as MF and Multi have said, there is nothing wrong in letting a bus pass if you can do that safely.

Exactly. You don't have to take primary position all the time; whether you do depends on your risk assessment of the situation. skydancer gives the basic rationale. Approaching a pinch-point? Take primary position. At a junction where you're at potential risk of being left-hooked? Again, take primary position. On a country road with consistent width and other road characteristics, no/few junctions? Take primary position when you feel it's necessary, but mostly you won't have to. Before taking primary position, during, and after, communicate as necessary so other road users understand your intention and the reason for it.