In article , [H]omer
writes
The size and shape of the grass stems has changed over the last couple of
years. They used to be dark green and very thin, but now they are thick
and flat; a lighter colour, and sort of horrible and gnarly. The whole
lawn is thinning out, with just these mutated stems growing in clumps.

Should I just give up, start again, and re-turf, or is there another way?

Have I been invaded by the dreaded couch grass? It doesn't have the same
long white root system, but it is just as ugly.

No, it's not couch (Agropyron repens) Could be Yorkshire fog (Holcus
lanatus) - very pretty en masse in flower, but that's not the effect you
were aiming for.
--
Kay
"Do not insult the crocodile until you have crossed the river"