You would need a extra deep well socket to fit the hext nut down in there deep. I don't know off hand what socket size is needed though.
Now you could fabricate one if you have access to a welder. You first figure out what socket size you need with a deep well socket. Then you weld on a short steel pipe to the socket and then another socket welded into the pipe end so you can use a wrench on it.

Here in the USA I used to look for a Snap On Tools or Mac's Tools dealer (they used to have mobile tool trucks go around and visit all the repair shops around town). Then order and or buy the special tool I needed from them. But I haven't seen a Snap On or MAc's tools truck van in a long time now. SUNEX for example makes a 1/2 inch impact driver extra deep well socket set, but it is a bit pricey, if you only need one socket out of the set. So fabricating one using the socket you can get more easily is probably the way to go.