Do you have dogs? My neighbour has dogs and when the wee on the grass it starts looking like your picture. Doesn't completly kill it, just makes it go patchy.

Is your soil heavy clay, or light and sandy? And are you more or less in drought in your area? How old is this lawn? And when did you first notice brown patches appearing - last autumn? Or early spring, or over winter?
This assumes you don't have a dog...

Hi there and thanks for your answers.

I do have a dog but she pees in a different area and I am positive this is not her work.

Our soil is a heavy clay and we are in drought with a hosepipe ban having just come into force this week.

The lawn has been here over 45 years and I have reseeded in patches before when the rabbits have done their worst and also spread a fertiliser on the lawn annually. Up until now, I've never had any such problems until these patches appeared at the beginning of this year and have got progressively worse until they are now bald as you can see from the attached photographs.

First thing is to check whether its leatherjackets - you'll need a large sheet of plastic or something similar. Place on the lawn at the end of the day, anchor it down with something, then pick it up first thing in the morning - if its leatherjackets, you should see grubs on top of the soil. Helps if you throw a bit of water about first though if its bone dry, so a watering can job.
Second test is this (not for leatherjackets, something else) - try pushing a matchstick into the turf, not a wet patch, just anywhere its dryish. Does it go in easily, or is it quite difficult to push into the ground, requiring a fair amount of force, or worse, does it actually break instead of penetrating.

looks like the bitch to me .

it looks like the bitches wee to me . i bet this photo is taken from in front of your house . you can see just at the end of the photo the lawn looks healthier . the patches are very destinctive and your dog wont run all the way up the end for a wee . id of thaught that leatherjackets would of been all over bye nature . try keeping your dog off this area with chicken wire for a while and see what happens .

Hi Noseypotter, Thanks for your suggestions though I'd definitely say these patches are different to where the dogs pees. She goes in a particular area all the time and the patches are not the same as those in the pictures. T

Thanks Bamboo for your pointers too. I poured water on a patch of the affected lawn and covered it with polythene but there was no sign of any grubs the following morning. A matchstick also went into the ground with a fair amount of force but it did not snap....

its worth fencing some of this lawn of just to lose it from theequation if nothing else i think .

At this time of year, if your lawn is not compacted, you should have been able to gently push a matchstick into the turf with ease. As you've heavy soil, the problem might be compaction combined with drought like conditions.
Have you ever aerated your lawn and if so, when did you last do it?
If the answer to that question is 5 years ago or never, that is one thing I would suggest you do as soon as humanly possible. Do it either by using a spiking attachment to a mower, a lawn spiker roller, or by hand using a garden fork. I prefer the fork method because you can make deeper holes - insert a garden fork into the ground vertically (stand on it if you have to, and you probably will), then pull it out again at the same angle it went in, and do that about every nine inches. Yes, I know, its gonna take ages... then I'd dig up the top 3 or 4 inches of the bald patches, create a friable crumb surface, level and apply grass seed. Or, better, you can apply a liquid feed (not weedkiller/mosskiller) to the whole lawn, wait a week and then treat the bald patches as described. Don't use granular feeds if you want to reseed...