Just in case anyone is interested, I took the advice and sent the Seamaster to ChrisN from the forums (
https://cjnwatch.co.uk)
Amazing service, including some of the most detailed reports I've ever seen. And some fascinating insights too, including the discovery of a non-standard seconds hand (now replaced with an official Omega one).
Photo included below. Chris put on a new cystal which completely changes the clarity of the dial.
Watch time-keeping has improved to +3 seconds a day - can't argue with that!
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