Do you mean BK036 on chambers?
https://www.gunspares.co.uk/products/24724/TX200SR/
It looks like they have labelled it wrong ('cover plate screw' instead of 'cover plate'), as B1306 is also labelled the same, but the prices are different. If that is the correct part, then for £5 it is probably worth just getting a new one.
I have been experimenting with laser cutting some new washer/shims from nylon (W085 and W086) as I can't find replacements for sale anywhere. They are untested as yet. I tried PTFE first, but they only lasted about 10 shots before deforming enough for the groups to open up. Let me know if you would like some of the nylon ones to try, as I now have about 30 pairs.