It's happened to me in the past when we had bad winter glas has frozen solid in door so when putting window down glas remains up. Had this on rover and fords lol. I simply reapplied bonding and job done. I use bonding from local motor factor which isn't expensive and is used to hold on rear wings and body kits. Looking at the fixing in your picture it doesn't look like a genuine rover mg part, I have only ever seen white fixings on any of my rover mg cars. It also looks to be split on bottom edge.
I will look in my shed to see if I can find any and if I am in Cardiff next week I can look in breakers.