Sear picot screw & spring
Thank you for that advice as I looked at it from a different angle and guess what: "I'll be buttered on both sides!" ...the screw was far too tight in the bearing end opposite the thread, so I put it in my jeweller's lathe & polished it until it was a better interference fit. I was then able to screw it in albeit tight (I suppose as it should be).
Note that it was very hard to remove in the first place.
Then I fitted the sear & the spring as suggested. And, it worked! Simple really.
Now the trigger works a treat and it smooth to operate.
Thanks very much. :)