This is is what our new double car main sheet traveller will look like, after our old single car failed, spilling ball bearings all over the cockpit!

Update: went a different route , rather than coupling a second traveller car as in the photo, I bought a much larger and stronger traveller car, the Ronstan RC12613, which fitted fine onto the Harken track. I did also buy the rebuild kit HMR99 from Harken for the old traveller, seems it was enough of a known problem for them to create a fixit kit. I will keep that as a spare. The new car is pictured below to the left and can be seen to be much more substantial than the old one to the right.