I think the best advice I can give is that you should as soon as possible water the patch copiously, to dilute the urine as much as possible.

You could also try a product known as dog rocks, I think - the vet should have it. You put it in their drinking water and it neutralises their urine, so it doesn't cause so much damage - if your dog will actually drink the water containing it, that is.

Ive done lots of stuff on this and the general concensus of expert opinion is that there's nothing you can do. The only thing that can help is Gattina's suggestion of copious watering straight after the dog has urinated.

Stuff from the vets, changing the dog's diet etc etc don't work.

Too true. The only really effective thing seems to be lots of water applied to the spot within minutes of her "duty". Water applied later won't prevent the spot, but will help rhizomatous grasses to recover more quickly. Keeping the soil in good shape with organic amendments--and/or gypsum in clay--will help reduce the damage, and help it to heal faster.