Did a search on Minimail as I know he does used parts and found this:
S/H = secondhand
http://www.minimail....p?q=hydrolastic
HELPER SPRING - HYDROLASTIC . PRE 1966 . S/H
Product Reference: U.21A1566
£9.00
HELPER SPRING - HYDROLASTIC . REAR . 1966 ON . S/H
Product Reference: U.21A1806
http://www.minimail.co.uk/
thanks for that!