Any chance of a photograph? Drought conditions usually produce straw like grass which goes brown - the description 'yellow' doesn't usually spring to mind if that's the problem, but some diseases of turf produce this. How long has the grass been in place, and do you have a dog?

Thanks Bamboo.
The grass has been pretty well established, but there is now lots of yellow colour patches.
There has been the occasional visitor with dog, but that was more over the winter period ...I have stopped it over the last couple of months!!
Could that have been the damage, and how should it best be tackled ??

It could be from dog urine, though usually the yellow would have turned brown and dead fairly quickly, but it could also be leatherjacket infestation (have you noticed any birds pecking at the grass?) or even a result of snow mould (starts out with yellow patches which spread out, then turn brown). Establishing the cause is critical before remedial treatment is carried out. I'm assuming that your lawn is not made up of fine, luxury turf or grasses, but is a perfectly ordinary lawn.