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I suggest that you try Speedy Spares https://www.speedyspares.co.uk/
You may however have to wait a while due to this Covid19. I would suggest you drop them an email anyway as they may be able to help by referring you elsewhere.
No problem at all.
I know they certainly do Rocker Shafts etc as I have had them from there for about £30 I think.
Hope you get what you need eventually!