2001 75 Conn judder in reverse

I have recently acquired a judder in reverse gear, thought it might be corrected with the upcoming belt change and 150,00k service but not so.

Flushing additive was added during service, drove for a couple of weeks and fluid replaced; still no joy, There are no t4 warnings.

All other gears work normally.

From what I have gleaned here and elsewhere it sounds like it might be the reverse piston, some info suggests that this can be replaced without removing auto box, can anybody confirm this please?

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