Is this plant in a pot or in the ground? Is it the older leaves which are most affected? Do you water in full sunlight, spraying the plant? Sorry to ask so many questions, but its necessary.

It was in a pot but I moved it into the ground last year (good soil, drainage etc). No, it's the new leaves - there are no old leaves anyway as it's deciduous. New leaves are healthy for about a month then begin to go downhill. It's not frost damage (my first thought) because we had a good spring this year. I never spray the leaves and it's not too hot as it only gets the early morning sun. I think it's diseased in some way but cannot identify the cause. Photo attached to prove that it starts out healthy, and to repeat, this has only begun to happen in the last 3 years. It not only spoils the plant for the remainder of the year but is preventing normal growth.
Any thoughts you may have will be very welcome, because it was my mother's favourite acer and she gave it to me to look after when she moved into sheltered housing. I hate the thought of letting her down!
(PS. Sorry, new to this site & cannot work out how to add photo easily, so you'll have to believe me....)

By 'old' leaves, I meant old this year, as in ones that appeared early in the year rather than ones which have appeared in the last month or so. I don't recognise this, or at least can't give it a name - looks almost as if the brown areas follow the veins in the leaf - do they? Or are they simply random? I take it you've no sign of beasties under the leaves or on the stems? Do the browned areas drop out, leaving holes behind?

It could be a mineral deficiency...potassium or magnesium?!!

Thanks for trying to help. I don't get any 'old' leaves because once this starts, no 'new' leaves develop. The brown spots seem random and don't drop out, they just increase in size until all the leaves are brown and die until the next year when it starts all over again. I've tried insecticide, fungicide and increasing the acidity of the soil but not potassium or magnesium - I could try that next year, thanks.

If the browning is between veins, then try a shot of Epsom Salts - if its short of anything, its more likely to be magnesium than potassium in acid soil. Adding potassium can cause magnesium deficiency.

A sudden thought has struck me - Plane Anthracnose. This is a fungus which causes leaf blight like this - it appears on leaves which were previously perfectly healthy, but the fungus is present already on the plant, evidenced by a small, brown discoloured paatch which develops at the base of a leaf bud in winter. Not very noticeable, of course, but affected buds die without producing a leaf. Adjacent areas of stem may turn brown and die back and small cankered areas may appear on the stem. Any leaves which open up and look healthy then develop dark brown necrotic patches, primarily adjacent to the main veins. If it is that, and I think it is, there's no control available, but its thought the fungus overwinters on fallen affected leaves. Suggest you clear away all its leaves in autumn and early winter, and I'd repot into a clean pot with new compost once its dormant, inspecting the stems thoroughly then, and later in the winter and early spring, for signs as described above. If you see any, cut those areas out. Contained in a pot, you might have a chance of keeping the infection at bay - there's no guarantee the fungus which causes this doesn't overwinter in the leaf litter layer which is always present on the soil if its in the ground.

I forgot to say that this usually affects Plane trees, but I can't think of anything else that fits the bill. There seems to be no suggestion anywhere that it can affect other plants, but then, on the other hand, I've seen Oak Gall Wasp affecting a Photinia growing beneath an Oak tree with the gall wasp, so who knows.

It may also be sunburn, if it was planted in a sunnier spot than it had when it was in the pot.