Thank you for your reply, Barry.
The frequency during the day is about 20 times a day, but is not as severe as at night when it can be 50 times or more. Normally I end up after about 4 hours going to sleep out of exhaustion. But I can feel the bladder isn't empty during the day either. I can never empty it.
I have cut out caffeine entirely, and mainly don't drink alcohol either. I don't think those two substances help at all. Diet Coke for example seems to severely exacerbate the problem.
I'll take Tamsulosin for a month and then ask the doctor for a referral to a urologist if that doesn't resolve it.
I don't have haematuria (blood in the urine), as shown by urinanalysis strips, but the urine is orange at the time, and not straw-coloured, and it's orange however much I drink, so it is not dehydration. I've told the doctor there is something in the urine, as the urine strips show the specific gravity at the highest level all the time, but the doctor won't believe his and just told me I'm dehydrated.