Well I revisited the trigger on this. It still had a very heavy pull. I swapped a long sear, short sear and then put the original trigger piece in (that sits in the trigger blade) and it was still bad. I decided to take out both sears and then try it. It was ok in half the housing but stiff when the top half was put on. I tried a small spacer and that still didn't help.
What seemed to be the problem was that the legs of the trigger spring were catching on the grip screw bosses. There is not a great deal of meat there anyhow, but I filed them a couple of times to make things better.
One leg of the spring always seems to slip off the plastic bit of the trigger, but as the trigger was lighter (In one half of the housing) I left it like that so only one leg is acting on the plastic bit.
I had to fit a longer screw to adjust the sear engagement and the thing plinks ok now but the trigger pull is very long. I may refit my modified plastic trigger, which enables me to get a shorter first stage.
So I have learnt something today. Sometimes when you revisit old problems, you often find something simple that you missed or decide to do something that you had put off doing.
It is STILL a polished Turd.