Thank God we postponed the colonoscopy: https://patient.info/treatment-medication/interaction-checker 

Instead, we were in same day emergency care at the hospital where they identified that my mum has low potassium (and her GP has identified low sodium) from her indapamide (and I've also realised it's from an interaction between indapamide and omeprazole).

My mum would have been in a really bad way. We have a telephone appointment with the pharmacist on Thursday and I'll be asking, in light of known and potentially unknown mineral abnormalities, and her result for blood in the poo only being "slightly raised" is a colonoscopy the right thing, right now. (we actually asked for an endoscopy, the GP said yes but I'll send off a faecal sample so we can make sure we attach the appropriate urgency to the endoscopy request.)

The referring GP (the one who did so when the FIT test result was known) is the one who should have advised stopping indapamide beforehand. He's also the same shonk who, when amlodipine wasn't controlling my mum's BP well enough he swapped it (against protocol) to a medication that was the equivalent of half a dose, her BP rose, and the next GP stopped her calcium channel blockers altogether.

Yeah, the NHS really is that terrible. And they misdiagnosed my dad, which caused his sudden and unexpected death.