Hi wgregor. just noticed you are in Plimuff. Hope things are good - I am a janner relocated to Sydney
The system has far toom much friction. I reduced line size from 12 down to 10 or 9, with dyneema/spectra. Have a look at where the fixed luff lines exit the boom. on my boat
they were rubbing the cheeks of the exit sheaves. The leech lines go into a complex block system on the cars. When I pulled the cars out of my boom, I found the blocks had been twisted so that there was a crossover between the blocks at the clew end and the blocks on the car.
Note that the luff-lines are spliced onto 6-8mm mins on the cars - something to factor into a refurb