"Martin & Anna Sykes" wrote
They sound like Dog Lichens ( GW magazine Aug, pg 45 ).
snip

I still have the mag so checked the piccy but that's not it. This is a
distinct whitish substance, for want of a better word, level with the
ground. Ach, it's times like this I wish I had a scanner.

There's no buried wood, I know, because this area used to be part of an
arable field and was subsoiled and rotovated before it was grassed.

Kay's suggestion of a build up of mycelium seems a possibility. I've tried
googling for some info and turned up one site with a picture that
looks -vaguely- like my weird stuff although my patches seem a bit denser
and more on the surface. See this page towards the bottom - takes
a few moments to load:
http://helios.bto.ed.ac.uk/bto/microbes/basidio.htm

Looks like fairy rings may be the cause, and we have had some that look
like the toadstools shown. Seems you can water the areas affected with
dilute washing-up liquid if it gets to be a problem. I think I'll just
spike the worst bits with a fork and otherwise leave it to see what happens
for the time being. Don't want to make any fairies at the bottom of my
garden homeless!

It's amazing what odd things you find turning up.

Thanks everyone for the suggestions.

Sue