I've returned from the fly infested wilds in the middle of nowhere.
The man I was meeting showed me his arm which was covered in fly bites from last night. When he mentioned that he had a bite very close to his eye, I told him to make an appointment to see his doctor today. With Blandford fly bites, they produce large blisters which can weep, and it is easy for secondary infections to set in, so they have to be taken seriously. The flies are attracted to water features - they have a small lake in the back garden, and rather like mosquitoes, they seem to also strike at night when people are in bed. They may have to consider putting up mosquito netting over the beds.
I took the precaution of wearing long arm shirts, a jacket and tie (top shirt button buttoned up to stop any flies trying to get down the shirt), and long socks, so I think I avoided getting bit. Thanks for the insect repellent and the protective clothing - they may have done the trick.
One thing I found out is that Blandford flies are seasonal in May and June, so I will try to avoid my return visit until July.