Interestingly enough odour had never been a problem until the pop off valve and pump ran out of life expectancy. I put that down to it being a Lavac. They are simply wonderful. Anytime in the future I buy a boat if it doesn't have one it will very soon after get the keys.
New hose is definitely correct size.
It does appear to be the ribbing that's the problem. I've taken the pump/pop off valve and hoses into workshop at work and will attack on dry land tomorrow. Judicious use of hot water and brute force. We'll see what happens. Don't have a heat gun but that will come next if necessary.