The v85tt (at least the e5) is hard to completely fill the tank - it splashes back and bubbles and doesnt want to take the last litre or two.
So having read lots of posts about drilling an air hole in the filler neck to allow the air out and therefore mor petrol in - i thought i would try. Here are my findings - possibly only applicable to UK e5 models.
The fuel tank is MUCH smaller than the coloured covers. A the top there is a 4-5cm vertical extension .
The restricive filler neck is shorter than this extension
So drilling holes in the sides of the restrictive part will only allow another 50cc or so in 
This may only apply to the UK e5 2022 model
There is so much space under the tank covers that if the tank was steel we could have had a much more svelte v85tt or another 10 litres of fuel!