If its the SBK I usually use, and it certainly sounds like it, I wouldn't recommend your using it to kill nettles - get it in the soil and it sits in it for 6 months, poisoning everything else. Usually used to treat woody roots, such as tree stumps or brambles, when its applied to cuts or holes made into the roots. I'd have thought a better option would be a glyphosate based spray such as Round Up. Remember to leave some nettles somewhere though - they're the larval food plant for some butterflies.

We also have a small bluebell wood and I just have a pass through a few times a year, pulling out the nettles, brambles, docks and so on. Get out as much of the roots as you can, accepting that you won't get them all. I burn the roots and compost the top growth.

You could try using a strimmer once a month.

If you use glyphosate after the bluebell leaves have completely died back you won't hurt the bluebells, but it will kill everything it lands on so your grass will be affected as well. With nettles you could try spraying lightly so the spray doesn't reach the ground, but this may not kill all the nettles. When you leave some nettles for the butterflies make sure its a patch in the sun as I have read they don't use the ones in shade.

Sometimes you can put on rubber gloves, and put cotton gloves on overthem, soak the cotton gloves in RoundUp, and wipe them on the nettles only. That should kill the nettles without damaging anything else.