[JulesJ]

I thought first maybe they were hoverflies, but they don't have the right colouring.  They're brownish/grey/nondescript. They're like small skinny houseflies - good, bad or indifferent? Should I be worried?

[paintedlady]

flies are rarely a problem as such, but it's a bit difficult to know from your description - if you are getting loads in the greenhouse, either something is attracting them or they've hatched from something already inside.  Personally I wouldn't worry about it