Thanks for all these ideas.
I am clearly confusing some of you with my previous references to sticking pucks (ie the puck sticking to the shore screen). This is not the problem since I changed to 18g IMS ridgeless basket which easily accommodated my 20g dose for our two morning lattes. On my other Elizabeth I have a similar VST basket and, as far as I can tell they perform identically.
This problem is only that the basket locks onto the brewhead sometimes when I fit the portafilter just snugly on the brewhead when the machine is cold and try to remove it when it is hot. Usually a wiggle of the portafilter frees it but my post a few days ago was prompted by the basket being so solidly jammed on the brewhead that it wouldn’t release until I levered it off with a teaspoon. It’s too hot to try to remove it by hand and wiggling the portafilter wouldn’t release it.
My conclusion is that it must be caused by the basket and brass disc being slightly non concentric so that if the disc is solidly locked by the central screw the basket will jam if it is rotated slightly so that the eccentricity locks it in the groove between the brewhead and the brass disc.
I hope that by only tightening the central screw just snug it will allow the disc to shift slightly to maintain concentricity with the basket and allow even clearance all round.
I hope this rather lengthy explanation helps to clear up any confusion about whether it is the puck or the basket that is the problem.
If anyone has advice about the effect of any leaking between the red o ring and the brass disc due to the fixing screw being too slack I would appreciate it.