So after a (£60) trip to the vet the toad concerned is now on a regimen of diuretics and antibiotics. I'm to give it, orally, 0.5ml daily of both: frusol solution (50mg/5ml per ml) and metronidazole suspension (200mg/5ml per ml).

The vet reckoned there was no hard mass inside the abdomen so it's more likely an infection and/or kidney/liver trouble. He said if the diuretic doesn't stimulate it's kidney and get the swelling down in a week or so it's probably renal failure. He also said he'd rarely seen a toad so long-lived, so its habitat and diet are probably just fine. He gave the toad the diuretic; told me to do the antibiotics myself.

On the way home from the vet, having had its frusol diuretic, it managed its first bowel movement since being quarantined. In all likelihood, it's not the impacted colon I'd feared! I'll update this thread if the patient makes progress (or regress).