Want to try the Double Reverse/Ravenna technique and need the lime putty? This site is the best I have found in the UK with a reasonable delivery price. A 15kg tub is £12.00 plus £7.00 delivery. This size will last you for a while.
They do 5kg and 10kg but the delivery price is the same.
For reference, this is non-hydraulic lime putty, it sets on exposure to air. Cement powder is refered to as a 'hydraulic' mix, i.e. it will set when mixed with water.