To the rider involved in an accident outside C4 this morning

This just pisses me off, the fact is that by cycling at 20mph you are going at a speed within the law (...)

Yes and no. The speed limit was designed for the motor vehicles.
It's more about the general awareness. It's exactly the same problem, as when let's say, you attempt to plough through a stationary traffic between the cars and a scooter zooms past you when you try to squeeze between the cars - sometimes you just don't expect anything there. But you should.
Pedestrians also should, but they usually feel protected while on zebra or pelican crossing, you see...
That's why as a cyclist, you should take care while riding fast next to a stationary vehicles stuck in traffic - not many people will expect anything coming at them at 20 mph.