Awkward bolt

Hi there.
I was wondering if somebody could help me please.

I am currently trying to remove the catalytic converter from my 56 plate 207 1.4.
There is 1 completely inaccessible bolt that sits inside the little u bend shaped bracket at the bottom of the pipe.
I will try to upload a photo.
I purchased short headed Allen keys but no joy.
Spanners won't grip it and there is no room inside the bracket to manoeuvre a socket.
I have removed every bolt apart from this one!!
Does anybody know how and what tool I need?
Please help save my sanity.
David.