Having just had the same problem, I had a real fight when refitting that ring! Didn't have too much trouble removing it, but making sure that it was correctly aligned with the tank top threaded part, while pushing the tank unit down (due to there being some spring loading on it) it was a bit of a mare!
I actually oredered a 'universal' tool from Aliexpress, and was pleasantly surprised that it wasn't made from toffee!
It's a useable item, and only around £7 or so, including tax. Takes about a week to deliver though. 

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