Hi David, welcome to GoY, it looks as though you have a bit of a drainage problem, try forking it all over the problem area, rake out as much as you can, if you can't get it all, you can use a fungicide, and don't cut the grass too short.
It's a bit late now for a spring feed, but in the autumn you can treat it with an autumn feed, Derek.

Can I ask what your lawn care regime has been during the last five years or so?

Many thanks for responding. I confess that my lawn care has been restricted to mowing. I will try forking and raking and, if necessary, using a fungicide.

Frm the pictures, although its hard to tell, the growths there look like small dog lichen. This occurs in lawns which are badly maintained, which is why I asked about your maintenance regime. If you're not sure what you should be doing and when, get yourself a copy of D. G. Hessayon's The Lawn Expert - it's only about a tenner brand new, but you can also find copies in libraries or second hand. The chemicals mentioned in an old copy might now be out of date, but the basic methods remain the same.

Many thanks Bamboo. I'm pleased to have a possible identification as nobody seems to have seen it before. I will try to get hold of the book you recommend - and try to improve my maintenance from now on.