I think it's definitely an arum lily. I've found lots more, which are flowering and look exactly like this.
Good to know that they spread like buggery - we will take quite a bit out. My husband does enjoy destroying the garden. He kicked a Rowan tree down the other day. In his defence, it was a reasonably young one planted far too near to the house.
Since I posted I've been investigating (only bought the house in August, and having cut back the masses of grasses now all the spring flowers are coming out) and have found asparagus, tons of wild strawberry plants, something called corn chamomile, masses of hellebores, fritillaria, aquilegia, forget me not, giant cowslip, what I think is goldenrod, lots of quite unusual tulips, oak leaf hydrangea, black lace elderberry, lots of iris.... we bought it from professional landscapers and it appears they have used the garden as their lab
There are simply masses of plants I have no clue about. Which means there will be more photos!!