I have a 1997 K and only this week changed all hoses to silicone but still struggled with the "U" even though the silicone hoses are more flexible.
I was on the point of calling on a stronger neighbour when it finally went on with the aid of washing up liquid and all the strength I had after trying for about 15 minutes, working from above, and fitting at the thermostat first to allow more movement.
I still have a new spare bought from Caterham about 5 years ago and it measures 30mm at both ends. The old one, just removed, has stretched to 33mm at the thermostat so it appears that brute force and lubrication is the answer.