Apologies for my radio silence glad you got this sorted I have zero confidence in most UK retailers and warranty claims but it is known as Rip off Britain!
That washer/insulator/gasket is the one that has gone to atlantis on mine

rendering my mini unusable as it is nowhere to be found. I was going to get someone to make one of the same dimensions out of delrin/peek/nylon but that is going to be tricky.
After reading your post I may well just remove the 510 completely and retrofit a FDV V4 510 that way any probs later on will be easily fixed with avail spares.
The 510 is epoxied onto the aluminium chassis I have to say one of minis has been the mod from hell partly my fault and partly design and materials.
I will post back when I get my life back.
You can see the missing insulator/gasket Technonut has posted about in the PIC below.
Does anyone know a good way to remove epoxy resin will heating it up be the best way?