Could anyone tell me why these patchy circles have suddenly appeared on my lawn?

No photo showing yet.

Two things occur to me - either a dog has urinated on the grass or there is some kind of fungal infection, most likely fusarium patch. If you Google that you'll discover how to treat it.

It said error file too large when I tried to upload the pic? It’s just a normal sized one I took on my phone.

Could be fungal but it could be a physical thing. With the dry weather it could be there are shallower patches of soil under the grass and these areas dry out faster. so the grass looses water and goes brown. or the lawn mower if its a rotary bladed one has cut the grass shorter in these places.
try watering the grass and see if it greens up.

Looks like grub damage to me. They feed on the roots until that part of the lawn dies. Then they move to a new and repeat. There are several products available to eradicate the situation. I'm in the US, but you should inquire at your local garden center or hardware store.

Fortunately, I never have to worry about grubs or slugs because the birds around here gobble them up in short order. The starlings seem to have 'radar' and know exactly where to find them.

Oh yes Bathgate, forgot about chafer grubs - will ask about them at the garden centre this week.

Might be as well to ask for a fusarium patch treatment while you're there. I don't really think its that (compare the pics in this link) but it can't do any harm - you just water it on.
https://www.turfonline.co.uk/blog/fusarium-patch-disease/

Thanks Yorkslass.