A fly(?) pollinating lady's slipper orchid

A fly(?) pollinating lady's slipper orchid

Observation - A fly(?) pollinating lady's slipper orchid - UK and Ireland. Description: I saw this fly within the pouch of the orchid flower and watched it effect pollination by climbing up the 'escape route' at the back of the slipper. Cypripedium calceolus is supposed to be unable to self-fertilise but this flower has subsequently produced

I saw this fly within the pouch of the orchid flower and watched it effect pollination by climbing up the 'escape route' at the back of the slipper. Cypripedium calceolus is supposed to be unable to self-fertilise but this flower has subsequently produced a seed pod (none other non-pollinated flowers have). I would like to know what the fly is.