House Flies?

Observation - House Flies?. Description: This is an observation I made some 15 years ago in South London so I hope you will forgive the lack of a photo. While taking down some velvet curtains for cleaning I was somewhat horrified to find a colony of large flies, the like of which I had never se

This is an observation I made some 15 years ago in South London so I hope you will forgive the lack of a photo.
While taking down some velvet curtains for cleaning I was somewhat horrified to find a colony of large flies, the like of which I had never seen before or since.
They were the size of green/blue bottles, and had very large round pads or suckers for feet. They were all light green in colour though not iridescent and their wings were all far too small to enable them to fly.
Any ideas please?