I just thought I should  report the outcome of my tap problem. After opening it up again, the first thing that was evident was a circling that was in the correct position but seemed to be loose. A quick poke made it fall out of its groove and on inspection it was badly worn.
  I do not have any spare circlips so I spent about four pounds on a selection box full. The replacement was in position a few minutes after the box arrived. The result is an almost perfect operation, not perfect due to some other worn thread as some of you suggested. Here are a few images of the bits involved. Thanks everyone.
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