Just as a follow up to this, I took advice from our local Selden agent. The first step was to ensure that all the nylon wheels on the in boom sliders were present. Two were missing and duly replaced. The inside of the boom then got a good flushing with a pressure washer. I bit the bullet and replaced the existing reefing lines with 10mm Dyneema – one size smaller diameter than the existing lines. Replacement was actually very simple, I just stitched the end of the new line end to end with the old one and pulled the lot through. The luff lines needed an eye splice, and some mousing line, but no great hassle really.
The long and the short of it is that shaking out a reef is much simpler and safer – no more standing at the end of the boom pulling through the lines. The sail goes back up with only a little resistance which is easily overcome by putting the halyard on the winch. I am now much more ready to reef, in the knowledge that unreefing will be simpler than before.