Well, after the discussion, I just did it. Hired the jig and the router from the local tool hire store, bought a brand new blade, and set to work.
The worktop was a U formation, and the main problem was that the base of the U was not at right angles - one corner was 110 degrees, the other was 85 degrees. Old Somerset houses are not built square!
The trick was to cut in the worktops on the two walls of the U first, then to rough-cut in the worktop at the base of the U leaving plenty of overhang for potential mistakes!
Setting up the jig simply requires an absolute 90 degree , regardless of the back angle, on all cuts. Provided you have a carpenter's square, it's easy enough to get. You pencil in the right angles AS IF the walls were square, then set the jig to these lines. It meant that I had to remove the stops on the jig, but that wasn't too much of a problem.
Measuring, and getting the angles absolutely at right angles is the key to success.
Once the right angles are pencilled in (I used white paper masking tape on the top of the work surface so I could see it easily, and to protect the laminate), I removed the stops of the jig (they would have set the jig to the wall angle, not to 90 degrees), set the jig by measuring the width of the cut from the edge of the jig, then did the cut.
The first cut was slightly out, but only because I misunderstood the cryptic markings on the jig. Having allowed plenty of overhang for mistakes, it was easily rectified in a second, more accurate cut.
I now have beautiful mason's mitres on both joints, neither of which are on right angle walls. Once you understand the jig, it really isn't as difficult as one might think.
But it does require VERY careful measuring, and you cannot afford to say 'I THINK the jig works like this'. You HAVE to understand the way the jig is set up. For example, the length of the dogbone cuts are set to the length of the standard bolts - and they are marked on the jig. But you have to realise that first!
Once you understand the jig, the rest is fairly straightforward. As I said, I used a new cutting blade, and took only 10mm at a time, thus keeping the blade sharp. I found it also helps to make the cuts fairly fast - not going slowly as one might think - so that the blade is kept fairly cool. There are two exceptions to the fast-cut - when you are cutting through the laminate, and when making the final smooth cut.
Be prepared for VERY large amounts of sawdust - after five cuts, the kitchen looked like an old fashioned butcher's shop - at least half an inch of sawdust in the cutting areas, and fine dust everywhere. If I could have done it, I would have cut outside!
I would not describe myself as an experienced DIYer - and this was my first kitchen installation. I had shown the drawings to an experienced fitter, who didn't like the angles at all. The quote was £100 a corner. £200 for making the joints was too much, which is why I decided to do it myself.
The hints and tips on this forum were a huge help - without them, I wouldn't have attempted it
I'm glad I did. The finished result looks great, and I am doubly proud of the difficult angles.