sounds like you have a wooly aphid problem. Since they are on the
undersides of the leaf, you have to make sure that the spray is
reaching them all or they will just repopulate. What "various sprays"
did you try? im not sure what chemicals are available in the U.K., but
malathion is the old standby for aphids, also anything with acephate in
it, or even oil sprays(read the label about temperature limits) will do
the job. You could even spray them off with a stream of water.