Spoke to Calor on the 'phone this morning they confirm their list price for 
a 6kg Propane refill is £16.07 inc vat.
They allow dealers to sell at what ever price the dealer wishes.
As regards the £18.50 price I was being asked for Calor told me £18.52 inc 
vat is the list price of the 6kg propane Light cylinder refill.    I doubt 5 
dealers all got the price wrong and were going to sell at wrong product 
price, they certainly did not put the light cylinder in front of me.

Anyway I've got a Flogas cylinder and have ordered from them online £13.64 
in vat and delivery!
To have the Calor delivered it would have added £4.50 to the £16
So I now know what to do in future.