...an issue of courtesy as the poor chap might be stuck in sideline in slow moving traffic.
The bike isn't "slow moving traffic". The bike can generally make headway quite nicely, as its rider hasn't chosen to join the popular Critical Mass style choking of the roads that drivers participate in in most cities during rush hour(s).
I leave plenty of time for my journey, am a nice bloke, and don't filter quickly, so I tend to let people in, if its safe. Sometimes I don't if I'm making good time, or have a cyclist whose behaviour I can't predict following close behind.
Note also, as the Highway Code says, that if ceding priority can help to avoid an accident, you should do so.
Motorists in other lanes will slow down when a car in a lane closests to the curb stops in slow moving traffic and as most anticipate that it is courtesy leeway that is being offered for those stuck in side lanes. I am sure most cyclist do that as well.
Depends. In situations where one lane is slow, and one fast (some filter lanes, for example) drivers in the moving lane seem entirely happy to ignore their suffering compatriots trying to join their lane. I see this frequently in some parts of Manchester and Crewe. Letting people in is more common where a single lane moves slowly, or all lanes move slowly.