I have been useing a 13mm ringy for the last 12 years on my original '97 VT 6 sump plug !
Its 19mm, I bought a spanner and it fits fine although I rounded it to hell cos it its hard to keep straight.
It took about a hour and a half being gentile with it then after getting frustrated I used some force and it slipped off every time, I was going to go look for some multi grips but I didnt want to re-use a rooted plug so I called it a day then went and bought a new a new plug for tomorrow. Including going and getting the stuff needed from the city its 6 hours for nothing today.
Last time was easy though with 2 people, one to push the spanner on square and the other to turn it.
I cant work out why the sump plug only has about 3mm of head on it and spanners taper in 2mm before its the right size, so trying to un-do it with 1mm of the spanner making contact with a nut that is soft as lead.
Anyway, a bit pissed with todays effort but tomorrow should be quick if I can find some multi grips.