by British standards its a reasonably large orchid! Your plant is the Southern Marsh Orchid - Dactylorhiza praetermissa. In southern and central England this is the most frequent of the Marsh Orchids, although less common than the Common spotted orchid with which it will hybridise. The marsh orchids have hollow stems unlike the spotteds.The shape of the labellum - the lower lip of the flower, its colour and the nature of the darker markings on it are the main things that you look for initially when deciding what you've got. This looks absolutely typical - very often you find hybrid swarms which have an amazing mix of characters and can throw up some very vigorousand extremely pretty plants.