Hello. When taking an antibiotic for a urinary tract infection (UTI), you should see some symptom relief (e.g., fever resolution, decreased urinary and back pain) after three full days of medication treatment. If you have taken the antibiotic for three full days without significant improvement, then I would advise calling your prescribing doctor about a change in antibiotic therapy. If your doctor sent a urine specimen for a bacterial culture test, then such a test may also provide information about which antibiotic therapy would be effective.
I realize you are in the UK while I am in the USA, but by comparison, trimethoprim would NOT be a first-line choice in treating a UTI here in the USA.
You might just call your doctor to request a courtesy change in prescription be called in to the pharmacy.
If a urine culture was performed, ask if it indicates a better antibiotic choice.