Looks like Osmunda Regalis, Royal Fern, to me. Have a look at this pic.
http://pagesperso-orange.fr/mon.jardin.promenade/osmunda.regalis.jpg

It can grow up to 2m high in ideal conditions

Bertie, It's not an Osmunda I'm afraid, I have masses of them here are the arrangement of the leaflets is very different from those in the pic....

I do recognise this plant, I've seen it growing as an escapee in Madeira, but I can't for the life of me remember what it is......not very helpful I'm sorry to say !

I don't want it to get any bigger than this. I am hoping it is full grown.

There's several growing in the forest around here with very similar leaf shapes, so perhaps I'm wrongly identifying them all as o.r.
Will be interested to hear what this is when someone positively identifies it.

Somebody help me and Bertie.

I'm reasonably sure that it's Onoclea sensibilis - which I should have remembered since we have it growing here in the garden! The fronds are highly variable as to the degree of incision/dissection, but are also quite distinctive. The individual fronds can be quite large - to 3 feet or so, but the plant itself has a rhizomatous, slightly creeping nature, so never gets very substantial.

Looks like the Onoclea sensibilis to me, judging from :

http://www.deq.state.mi.us/pw/images/Onosen.htm_txt_Onosen.gif

Well done Ilex.

Looks like a very interesting plant. I see a French nursery jardindupicvert.com sells it, and it looks as if it would be a good plant for a shady pondside. I shall order one now!

Thank you gentlemen very much. I wonder why 'Sensibilis'? Must be a sensible fern and not a silly one.

Bertie: It is a lovely thing, we have a clump growing away happily in quite deep shade beneath an Acer palmatum.

Jonathan: the name actually means "sensitive" and comes from the observation by early American settlers that it was very sensitive to frost, the fronds dying off as son as they were touched by it, despite the fact that the plant itself is perfectly hardy.

I do like it when the name of a plant has a good story to tell :-)

Makes good sense.