Tightening back nut

[Best]

Could you post a picture of the underneath of the basin showing the recess etc?
7" Footprint grips are a handy tool for awkward jobs. The type with knurled nut giving full adjustment are excellent

 

[buckley plumb]

http://cdn.toolstation.com/images/141020-UK/800/16973.jpg

 

[Best]

I thought the OP was talking about the single large nut type.
If it's the nut & bolt type, it's easy

 

[buckley plumb]

I thought the OP was talking about the single large nut type.
If it's the nut & bolt type, it's easy

I am not sure myself a pic would help .you could be right it could be a monoblock with the large back nut like a normal tap.

 

[buckley plumb]

http://cdn.toolstation.com/images/141020-UK/800/43610.jpg

if all else fails get some onto hold the tap and tighten nut with hammer and blunt chisel .