From a USA perspective looks like a mealy bug infestation.From my entomological unfamiliarity across the pond it would be some kind of unarmored scale insect or woolly aphid infestation. In any case other GOY members in the UK would best be able to tell you what is available to you for control or eradication.

Close up image would help.

it is the remains of a female mealy bug after her offspring have eaten her away. they youngsters have escaped!

on of the trees at work is covered in it.

Thank you Loosestrife, Owdboggy and Seaburn . . . I showed my gardening friend this morning, and she thought it looked like some kind of scale. If Seaburn is right (as she usually is!) then probably I can just try cleaning the leaves.

Sorry Owdboggy, I did take a close-up but it was too blurred so I didn't put it up.

I'd also spray the foliage with a systemic insecticide . the little fluffy blighters will room away and sap suck and grow larger. Certainly clean off the larger blobs of fluff. when you squeeze it a brownish goo oozes out squished bodies Yum yum :o)

Eaten her away?? Nature can be so distressing at times!

Thanks again Sb!

Don't be distressed. The female mealy bug lays her eggs-approx 50 to 100- and dies shortly thereafter, her job being done. I might add that there are some species where offspring are liveborn. This insect is of the order Hemiptera which means it has stylet type mouthparts and the mealy bug feeds exclusively on plant sap. Follow Seaburngirl's advice and apply an insecticide though in this case with an unarmoured scale a topical one might be adequate. In the case of an armoured scale such as the San Jose Scale (USA only) systemic is the best way to go.