Get Dremel on the bolt chop some off thread then get socket on it.
Using a screwdriver isn't a option on the tiny little bit of slot that is at each side of the nut you'd probably have to knock it round with a hammer maybe bending or breaking something in the process.
Can't really tell how tight it is in there from pic but you could chop a old 10mm spanner down to get in the gap and just nip it up best you can?
Or you could just undo clips on underside of sink/workshop, lift sink out tighten nut up cutting some thread off bolt at same time for ease next time it may happen.
The bolt isn't fixed in to the tap though it's a stud and that bit may have come loose. Not that it makes a difference just wasn't sure if you knew or not?
Leigh
Using a screwdriver isn't a option on the tiny little bit of slot that is at each side of the nut you'd probably have to knock it round with a hammer maybe bending or breaking something in the process.
Can't really tell how tight it is in there from pic but you could chop a old 10mm spanner down to get in the gap and just nip it up best you can?
Or you could just undo clips on underside of sink/workshop, lift sink out tighten nut up cutting some thread off bolt at same time for ease next time it may happen.
The bolt isn't fixed in to the tap though it's a stud and that bit may have come loose. Not that it makes a difference just wasn't sure if you knew or not?
Leigh