At the end of the day carefully perusing eBay yielded me a new, unused, old stock clearance genuine Honeywell valve complete sub £20 A couple of months ago.
You might be wasting a lot of time and effort on an old valve with worn drive parts in the head.

On Tue, 2 Apr 2019 23:36:06 -0700 (PDT), Cynic
wrote:

At the end of the day carefully perusing eBay yielded me a new, unused, old stock clearance genuine Honeywell valve complete sub £20 A couple of months ago.

Ok.

You might be wasting a lot of time and effort on an old valve with worn drive parts in the head.

Oh, indeed. I'm not particularly fussed to repair the MV, just that I
might if it's easier (not having to drain down etc), once I know where
the issue is etc!

Cheers, T i m