Thank you for all the replies, folks. They're really useful.
I haven't asked to stop taking Bendrowotsit. I simply stopped a few weeks ago. Since then I've done three patches (about five days' each) of BP measurements at home at different times of day. As well as apparently not making any difference to my BP, I didn't notice any diuretic effect. However, I do notice the diuretic effect of lettuce and cabbage, which I've begun to eat more of as they also help with the constipating effects of the painkillers I take for arthritis.
In a little over a week (when I'll need another prescription of the other BP med I take) I'm going to report back to my GP the reasons why I stopped taking Bendro, namely the bedtime cough it seemed to be causing, which could sometimes be quite distressing, and the painful swelling of my feet and ankles in the evenings, and send him the BP readings.