Here's what I would do, tilly:
You need:
25g/1oz butter
2 small shallots, very finely chopped
1 stick celery, very finely chopped
½ glass dry white wine
750ml or 1.25 pint vegetable stock (can buy but easy to make)
1 bay leaf
150ml/¼ pint carton single cream - or nat. yoghurt if on the cal reduction or as a compromise, half and half crem/yoghurt
2 x 85g/3oz packs watercress, chopped
Melt your butter in a saucepan and gently fry off the finely chopped shallots and celery for 2-3 minutes until tender.
Add the wine, stock and bay leaf and simmer for 2-3 minutes.
Remove the bay leaf, pour in the cream or yoghurt and chopped watercress, heat through, season to taste and serve.
Dead easy and delish