patchy lawn

Welcome to GoY, Mary!
What do you mean by "dark patches"?

Are the dark patches where you reseeded?

Stergram and Tugbrethil thank you for your messages. The patches are not the reseeding, they look dry but very dark. There are three patches in all. It almost look as though we have poured oil. This sounds silly but it's hard to describe.

Have you tried watering really well? Can you tell if the grass is still alive? How big are the patches?

If it was in Arizona, I would have thought that the soil had a high sodium content. Locals call it "black alkali". Is this anywhere near the seaside, Mary? Alternatively, areas that are having trouble with too much shade can develop a black-green stain from algae, or juvenile mosses.

This might be a silly suggestion but could your mower possibly have a slight oil leak?

I live in Lancashire England so not drought conditions here just lots of rain, the mower we have is electric, so no oil but thanks for your suggestions.

Another suggestion that's just occurred to me. Did you notice any mushrooms on the lawn last year? Sometimes they can cause the grass to look darker than the rest, as in "fairy rings". Keep a look out this autumn and see if there are any.

Hi Stragram thank you for your observation and suggestions. We have decided to get the experts in to advise. Many thanks

Well, most of us are experts, of one stripe or another, but it's always better to have a man (or woman) on the spot, who can actually see and touch and smell the affected area!
Once again, Mary, welcome, and feel free to call on us anytime!

it would be interesting to hear what the experts tell you, if you could add another comment to your question when they've been please?