Interior non camping Tailgate strut lock

Gas struts can be repaired or re-engineered by these sgs-engineering http://www.sgs-engineering.com/shop/categories/search/gas-struts

Brickwerks also sell replacement struts

Mocki: The copper pipe is soo easy to do, and will keep the tailgate up when the bikes are on that rack....

22mm copper pipe (doesn't have to be copper, but it's easiest to get)

Cut it the same length as the thickest part of the tailgater strut(the black bit) then cut a couple of indents in the ends of the copper pipe at the end and slide the pipe over the strut then when you lift the tail gate slide the pipe own over the thin bit of the strut and lock the indent to stop the strut shortening.