Dave,
We just kept offering a short piece (about 4") of the new bolt rope up to the slot to see what needed 'easing'.

To open up the feeder part of the slot we used a combination of a Stanley knife, round file, half round file and some sandpaper. Once we got the size we needed, I took off just a fraction more and we then made good with gelcoat filler. When dry, this was sanded fair and the job was done!

This was when we discovered that the slot also needed to be eased in a few places, so we used a broken drill bit (I think it was about 8mm) and wrapped it in sandpaper and slid it down the slot. It took a while but we just kept working away at a few high spots in both the slots until the sample piece of bolt rope slid easily along the length of the slot.

I suppose the whole job took a little over an hour and a half. Have fun!