So like most old defenders my steering box (NTC1582 6-bolt) is leaking and have an advisory on my MOT to get this sorted. Can I seek advice if A) I should tear down the box myself and replace the seal or B) send it off for a recondition unit and swallow £300 for the privilege? How difficult is it to do and do you need any specialised tools?

If B) is the way forward any recommendations on where to get a recondition steering box from?

I've had both good and bad experiences with rebuilt boxes and they aren't cheap
I recently replaced the lower seals (you can get a lower kit for the 4 bolt stc2848) on a vehicle I look after and last I heard it hadn't leaked and they can be changed with box in situ

I wouldn't want to change the seal in situ, you need to be quite careful pushing the seal in so not to damage it, although I presume it's possible.
It's not that difficult to take the box out, messy though.

Take the box out ... it gives you the oppertunity to clean/repaint around the area and replace bolts and washers + any of the steering components that look a bit bent/manky at the same time etc.