Well, I have scrubbed and it must have been mere chance that the first bit of gunge that I rubbed between my fingers was aphid free, because the others were absolutely full of nasties (reddish brown when suitably pulverised). There were some leaves which had gone very curly, and on close inspection these had hundreds and hundreds of beige rice grain sized blobs. I picked these leaves off and drowned them in soapy water.
I think the tree must have been affected by these things (or something else?) in previous years, because there are a lot of scarred bumps/galls on the old wood - do these things overwinter in their old hunting grounds ready to pop back later? Obviously we are going to keep a close eye on the tree now, so thank you both.