Hi Sjbt,
I had a very similar problem last year, following on from HDRBrachytherapy, which gave me a combined problem of a prostate infection and radiation induced cystitis.
Antibiotics cleared up the infection but did nothing for the cystitis.
What I found was that the majority of 'Cranberry Juice' on sale in the Supermarkets, doesn't actually contain much Cranberry Juice, if you read the labels carefully it's described as 'Cranberry Juice Drink' which is basically sugar, water, flavouring and a bit of cranberry juice.
What worked for me, was the real fresh cranberries, available in the supermarkets any time soon in the run up to Christmas. I would buy a bag, boil them down in a pan on a low simmer, and either eat them neat, or mixed in with a bit of cooked apple.
In a matter of weeks my problem had cleared up.
If you can be bothered to search back to posts a year ago, quite a number of regulars on this site followed my example and had good results with real fresh cranberries, so it's definitely worth a try, but use the real thing, not the ready made cranberry Juice Drinks.
:)
Dave