Tried heat, sat the hose end in boiling water for what I would have thought was a reasonable period of time , didn't seem to help cos it doesn't soften the reinforcing.
Hamish' suggestion re the 'barbed' fittings does make sense, cos it was the 'barb' that was giving me the most grief.
I think not all of the posters are talking about the same kind of hose --
There is an odor-free hose sold in the US made of a white plastic substance that has no reinforcing, but is still a bear to get on the fittings. But I believe it is compatible with barbed fittings.
I think wombat is referring to the hoses with spiral metal wire running through it. Probably not compatible with the barbed fittings.
I have redone the waste plumbing on a previous boat with the white stuff ($9/foot!) and it really does keep the odors out of the boat interior. I am in the middle of removing the hoses with spiral reinforcing from my recently purchased boat, and let me tell you, the odor was unbearable
until I got rid of them.
IMHO the only odorfree hose over the long term is the white stuff -- expensive as it is -- it should last forever.