I have a bitch of getting on for 16 years who developed PRA from 8/9 years of age & was completely blind by 11 years. I feel very sad about this
  & so when I get another dog ,that is in the future, I want to be sure to go to a breeder who has tested for this unfortunate disease. Can you
advise & help to point me in the right direction?  I live in North London.

I would only say that you can buy from eye tested parents, but this cannot guarantee that any puppy will not be affected.

PRA is a recessive trait and there are always going to be unknown carriers in the breeding population.  Their status will not be apparent unless they happen to be mated to another carrier, when statistically 1 in 4 of their offspring will be affected, and of course this could take years, and so they and their offspring will continue to ahve been bred from.

Half the offspring of a Carrier will be carriers and half clear, and you can't tell which is which.

It is fortunate that the incidence of this desease has reduced considerably since eye testing became available, but there are no guarantees.

I am sure you already know most of thye above having had a dog affected you will have reserched the disease.

Your best bet is to contact the breed clubs for a puppy list or breeder list of breeders who routinely eye test.

Lhasa Apsos eyes are by and large very good with GPRA the only hereditary eye condition to be tested for. When a dog has been tested the testing Ophthalmologist stamps the back of the KC registration slip so make a point of checking this. Also the test is a yearly test so make sure the date is within the last 12 months.

Unfortunately this is only of limited help because a dog developing this condition late in life could well have finished her breeding life before it manifests it's self. The same thing applies to late forming cataracts which affect some breeds such as Labradors (but not Lhasa Apsos)

At one time I would have recommended DNA testing but at the moment I do not believe that this is foolproof. Too many DNA tests seem floored.

Regards, John