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Ok I worked on the house and it is passing to swap the full valve would be very awkward. I guess I should really tell the customer? On the external stop tap the plastic on off piece is broke the water board is coming out to change that.
Unscrew the guts, as per your photo, inspect the valve seat.
If the seat is good, just change the black tap washer.
If seat bad, regrind first.
If handle very stiff or graunchy, swap out guts.
If seat too far gone will either have to swap whole tap - or leave it in position fully open and add another tap further up the pipe...