Wild Orchid

I have an old stone sink in my garden which has a mature collection of reeds. A new plant appeared recently which could have been a weed but as I didn't recognise it I left it alone. It has now flowered and is obviously identifiable as an orchid. I've been on line and it looks like a Southern Marsh Orchid - anybody out there know how I can ensure it stays alive? In the mean time I'm calling CSI South Wales to work out how it got there in the first place, I live in a built up area and I don't know of any local sites where it's growing naturally and considering the sink is the only permanently damp area in the garden I find myself saying - you shouldn't be here!!