99% sure this is Eriophyes sorbi mite.

Well it is not a Sorbus!

If it is a Rowan it Is a Sorbus. And that is Sorbus Mite Gall

Pimpernel look at the leaves... this is not a Sorbus!

MG..They look like Sorbus, What am I missing?

This is an early spring leaf.

It looks like a Sorbus to me - Sorbus sargentiana. I would remove all the infected leaflets though mites are more cosmetic damage than a dangerous one.

Thanks for identifying the problem....is there anything that I can do so stop it returning next year? I'm sure I've had this problems for about 4 years, and it seems to be already there when the leaf unfurls. I would appreciate if you could give me any pointers! Many Thanks.

I think Rowan is Sorbus acuparia-- the mountain ash

You're right Pamg - MG the name of the group of plants is Sorbus, and Rowan is just a common name for a particular variety.
Agree with Pimpernel's diagnosis - the mites come out of the leaves as winter approaches to hibernate somewhere - clear away all dead leaves from beneath and around the tree. There is no treatment available for these, but if you have only a few leaves affected, cut them off once you notice them next year. It won't kill the tree though, and don't remove more than a third of its leaves in an attempt to control the problem.

according to the RHS there is no effective chemical control of this.

Garden Hygiene is the best method burning all the affected leaves as soon as you see them. The mite eggs begin in the new leaf buds, that's why they become discoloured so soon.

I did notice that some American sites mentioned using "Horticultural Oil" on the leaf bud just as it breaks. i have no Idea what Horticultural oil is....maybe a trip to a garden centre or an email to a specialist grower will help.

They mean things like Neem oil, Pimpernel - the trouble is, not sure about availability in the UK, and for the purposes here, Adcordiner would need a spray oil which can be used on foliage - not all horticultural oils are suitable for use on leaves.

Ahh ! Thought as much, and spraying each leaf on a Sorbus would be tedious to say the least.

Actually, horticultural oil is a very light form of mineral oil, often emulsified for easier mixing and spray. Because of the ecological hazards of spraying petroleum products around, it has largely been replaced with various vegetable and/or fish oil products. Mainly, they are used to kill insect--or mite--eggs, or fungus spores, on dormant wood, or just breaking buds. Some forms of oil spray--called "summer oils"--are supposed to be good for spraying on open foliage, but I have found them likely to burn the leaves if the daytime temps go much over 75º F (24º C).

i agree cut them of as soon as possible and burn them strate away . dont compost them .