but all the ones I've seen are noisy and not very effective - they have
the fan heater problem of warming the room up quickly but then allowing
it to cool down just as fast - there's no thermal mass.

Hmm, yes. This is what concerns me, I definitely do not want the noise
a fan heater, nor the draught, but I can't help feeling that a 2 metre
long tangential fan rotating at a pretty slow speed could be quite
inaudible, yet seriously increase the airflow throw a radiator.