Any chance of a photograph of the brown blotches? Could be a leaf spot, either bacterial or fungal, but is there any evidence of anything on the trunk, stems, branches as well? And does the fruit show any evidence of brown rot or anything? Do you prune it regularly?

This happens to all our quince trees too. I think it's a problem of limy or chalky soil as you may have in your part of Wiltshire. It's most likely to be rust, though quince can suffer from fire blight too, which is a serious disease. Unfortunately we haven't found a cure for it (we are on alkaline clay) and the trees survive but we never get fruit. You could try regular spraying from early spring with a fungicide, though I haven't tried that yet.