I don't think the problem is with the safety spring. We've tried all different size springs in there, including a modern factory item, and none of them quite worked perfectly.

I reckon the trigger unit it's self needs a strip / clean / deburr / relube, as Mark from Hereford pointed out.

If I'd had my HW spares box with me, we could have quickly confirmed or discounted that theory by trying a different trigger unit in the gun.

It's annoying that we couldn't get it going there and then for you, Mel.