Thanks for the responses. It does seem strange to see withdrawn units in very good nick externally unlike my days spotting in the 1980's when there were some really grotty examples which were worthy of the scrapper!
Does seem to vary by depot. When the old LMS Merseyside units were replaced by the 507s around 1980, the Southport line trains were as described, with large areas of peeling paint, frayed moquette, jammed sliding ventilators, and even notable body dents. The comparable Wirral units were the opposite, and even when being broken up in Birkenhead looked fully presentable. Yet both had been overhauled at the same works with the same materials through their lives, and were likely on the same maintenance budget.
Presumably that was one reason why the blue paint on BR stock used to fade so badly
Blue was a colour whose pigment long faded more than others, and was regarded in the paint industry as "difficult", hence a preponderance of red and green in liveries. Of course, white is the worst. Paint technology at ICI came up with a more resistant blue in the early 1960s, which was a principal reason behind the new BR livery of the era going for it, as it was different to what had gone before (often the No 1 justification for new liveries). Actually it was not, inevitably, all it was cracked up to be, especially where repainted too thinly over existing colour (it worked better on new stock), and there were a couple of further paint pigment enhancements before it was finally felt to be adequate.