Hi all, A follow up to my post from last week -
I seem to have got to the bottom of this. Largely with thanks to Tony Irwin via The Society for the study of flies (Diptera) who have an excellent resource in their website here -
The swarming flies turned out to be Non-biting midges (Chironomidae) are also sometimes called chironomids. There are There over 5000 known species worldwide. Exhibiting a high degree of sexual diamorphism the males have hugely feathered antenae. It is extremely difficult to accurately identify an individual species so this is the best we can do.