Mrs Eggy,
I know it's not cystitis, but I asked Gnetters for advice on cystitis and I got some suggestions which may just help you. I've only just started on it, but it's called D-Mannose and the blurb says it's for UTI's and bladder discomfort. Take a look at
www.waterfall-d-mannose.com/shop.html
So far I've only taken four tabs, so it's early days for me. I too have given up tea and coffee, citrus and tomatoes among other things. I find camomile tea soothing.
Another product I've been taking which may just help you is a probiotic for women called Optibac, although there are others similar.
Has the GP mentioned a pessary of Estraderm? It's to improve the tissues of the vagina. As we get older it dries out and causes discomfort. It may be worth investigating.
I know how miserable it is to have persistent discomfort, hopefully someone will come up with a solution soon. Take care. 