Riding over the clwyds on weds and got well and trully acosted by flies every time I was going up hill or had stopped. The flies then followed me for ages until I could get up enough speed lose them (a lot of the route was uphill). I was thinking, after I had got away from them, do they return to where they were in the first place or do they look for another pile of sheep shite/passing walker/cyclist and are thus nomadic?

It sounds to me like they were probably Hairyarsus Pikeii flies.

And yes, they are nomadic…….. basically just bumming around with no fixed abode.

They are particularly fond of dog shit, although they can get distracted if they get a whiff Betty Swallocks riding past.