When I originally got the Disco (brought as spares or repairs off ebay without viewing first) it did have misfiring as the reason for sale. The previous owner had tried replacing the spark plugs and come to the conclusion it probabbly wanted a set of spark plug leads and I suspect felt out of his depth. I drove it back from the outskirts of Gloucester to Milton Keynes and it was fine as long as you didn't give it too much throttle though clearly something was wrong very much like what you describe. That turned out to be mostly his half-assed attempt at changing the plugs as 3 of them were so loose you could move them with your fingers, I replaced the leads at the same time as I didn't like the look of them at the same time, though suspect very little was actually wrong with them then.
If you suspect 1 lead is going bad, your best to change the lot. 1 cylinder logging misfires would suggest its more likely the plug or lead than the coil pack etc - these usually fail on a pair of cylinders, if you look at the diagram the top pair of the coil is one pair, the bottom two is another pair and so on. So if you had 2 on the same side of a single coil pack, it would suggest the coil is at fault (I'd suspect a damaged plug or shorted lead might cause the same result though as it would short circuit the good plug on that one).
Also changing the rest of the plugs given you know they are ancient would be a good plan, Might also be worth resetting the adaptives and seeing what your then left with, given the other logged errors. Once you've done that, have a look what the trims look like with the Nano would be the next step just in case something else is wrong (vacuum leak for example might explain some of the symptoms, particually if its not constant - something like a pipe rubbing against something so its only leaking some of the time)