I'm curious to think that there is a local troop...I've been doing something like guerilla gardening in and around Cardiff for nearly 30 years on and off, but it is a lone activity that makes other people nervous, "borrowing" seedlings that will clearly die in the undergrowth of other woodlands and giving them a chance in open ground eg on neglected embankments that the council leave to useless weeds like brambles and buddleia. At the present moment my nearby target is the north side of the steep embankment of the road that leads from the exit of Ferry Road Asda's petrol station down towards the River Ely. As is typical, large amounts of public money were spent on landscaping the park created on the old tip down there, and then the whole project was abandoned for lack of maintenance money after the scamsters had made their profits. So in all likelyhood it will be left to accumulate fly tipping etc and nobody will hit it with weedkiller or cut down the undergrowth - especially if it is an attractive piece of woodland. Feel free to join in and choose the sorts of thing that animals can eat and plant it all randomly, the survival rate is low on this dry south facing slope at the present moment so proceed philosophically.
Only saw this forum because I was writing a piece about Richard Reynolds book and listing the websites that he han't made a list of in the book, some of which I drew a blank on - seehttp://repwblic.informe.com/diggers-guerilla-gardeners-palwyr-herwfilwyr-garddio-dt213.html