Colin M - The only thing to watch out for is I would not recommend picking from busy road sides as nettles may have been sprayed with chemicals by the council (which can take some time to be apparent) and general contamination by vehicle exhausts.
Nettles are great, I use them to make not only soup, but also pesto, a nettle and Nori omelette and gnocci.
Anyway, this is my version of nettle soup

The first thing you need to do is collect younger nettles, but they don't need to be that small.
Do this re-using an old plastic shopping bag and a pair of good rubber gloves.
Then strip the nettle stalks of their leaves. The used stalks can be set aside and used as a fertilizer for the kitchen garden plot.
I had about 300 gms of nettle leaves for this soup, but the more you get in the better really.
After that, its a very straight forward and simple soup.
Sea coriander is a plant I forage locally, I don't know what it is called properly. The stalk and little green berry heads taste of lime and coriander.
To replace this ingredient, simply use a tablespoon of chopped coriander leaf, just as good.
INGREDIENTS
1 Litre lamb stock (or substitute vegetable or chicken stock)
300 gm or more nettle leaves
Knob of good Butter
Salt and pepper to season.
2 tablespoon's of wild garlic leaf green is you can find it
- or substitute 1 clove garlic, crushed or garlic chives - .
1 Onion, finely diced
1 Tblsn Sea Coriander berries
or substitute with finely chopped coriander with a pinch of lime zest
1 Tspn Horseradish, finely grated (more if you want extra sting in your nettle soup)
METHOD
Put the butter into a heavy based pan and heat up.
Add onion, garlic, horseradish, and if desired bacon, to the pan.
Cook until onion softened and translucent.
Next add the nettle leaves and sea coriander berries to the pan.
Keep stirring the nettles and reduce them as you would with spinach.
Try and give them all a coating with the oils from the bottom of the pot.
Next add your stock and bring to the boil, reduce and allow to simmer for 5-10 minutes.
Then blitz with a food processor, return to pan and allow to simmer for another 10 minutes
I would normally serve with some creme fraiche, but did not have any so I just used a small round of goats cheese on top and some soda bread on the side.