Entomologists to the forum…

What flying insect would be about the size and colour of a greenfly, in the Scottish Highlands and would bite a human enough for me to feel it and leave a prominent and itchy weal a couple of days later?

Hulk midge?

A greenfly, that happened to land on the same spot where something else bit you a second before but you didn’t notice at the time?

Loving the hulk midge

Not sure about the colour but that sounds like a sandfly. Experience them in NZ, they ain’t much fun, as they like to hang around your ankles, even after you walk away from the their enviroment.

Sandflies are normally tiny IME

Hulk midge?

Was it one of these?

Horse fly?

I’m loving some of the answers but none are correct. It was larger than a midge, green and had short legs.

I’m loving some of the answers but none are correct. It was larger than a midge, green and had short legs

There are a lot of nasty little insects out there, and to be honest I think that climate change has increased the range of quite a few that would never have been seen in Scotland. That said, the only thing that comes to mind from your description is Michael Gove

Birch fly is the other bastard of bastards

Ked / Deer fly? They’re kind of greenish and it’s the right time of year for them.

Aphids do bite people, rarely.

(Pedants will note it’s not really a “bite” but more of a stab.)