Central heating refill valve

In this house, we have an own central heating system. Periodically the water-pressure has to be checked and when found too low, you have to refill the system.
For this purpose there is a valve in the bathroom, at the lowest point, where a hose can be attached.
The only thing is the supplied wrench has been missing for at least as long as I rent the place, so I always use a wrench of my own and judging the scratches on the valve, the people before me probably did the same :)
It's not a real metric-sized tap and I don't have something like 3/8", so I use a 10mm one.