I hope someone can offer me some advice.

My hot water tank is factory-lagged. There is a leak at the point at
which a pipe goes into the coil, half way up the tank, and I want to
undo the joint there to see if I can seal it by using new PTFE or
something without having to call a plumber.

The problem is that the nut is recessed into the lagging with a gap
around it of about 10mm, and I cannot get at a purchase on it with any
of the tools that I have.

What sort of cranked/offset spanner should I be looking to borrow or
buy?

--
Tony