That is actually correct, you did miss something.
I thought I had.
If the slots were 30mm then the guide bush would probably jam in the slot at some time. The extra 1mm or so allows for a bit of slack to make sure this doesn't happen - this is exactly how a kitchen worktop mason's mitre jig works. If the router is going from side to side then I'm afraid it is you who is to blame. The cut running away from you, the first cut, needs to be made with the router pressed firmly against the left hand side of the slot. The return cut, the cut coming back to you, is made with the router pressed firmly against the right hand side. At the end of making the cuts the drain slots are sanded out to smooth them off and remove any machining marks.
It is really very straightforward and I've done this task quite a few times with both solid wood and Corian worktops. If you are having difficulty controlling the router then I suggest that you make a couple of lighter passes before going to a full depth pass as ball nose (cove) cutters used in these jigs do tend to deflect when plunged in to full depth.
OK thanks for the tips. Whatever you do, you need to be strong to control the router with big cutters and 40mm worktop even doing it the correct way. The forces exerted by the cutter are quite large. Exacerbating the problems was the matter that I wasn't cutting the whole depth but gouging out about 3/4 of it.
With the Imm gap, the router can easily tip from the vertical, if you decrease the force on it even slightly. And that worktop is expensive, even the bits I bought off ebay. With my tight-fit solution, the router doesn't tip nearly so easily in the guide. In fact I think it is a good innovation.
The jigs are designed for use with a plunge router only and are unsuitable fot fixed base routers and to avoid further damage I'd suggest that you retract the cutter at the end of a cut before lifting the router off the jig (the most common mistake made by people when using worktop mitre template jigs, for example).
Good ideas. Thanks.
BTW these jigs are manufactured from HPL (high pressure laminate, or a form of phenolic plastic) and can be easily repaired using Araldite (epoxy) which is filed and sanded after it has set (at least 24 hours). Even us pros get it wrong sometimes!
I was going to use high strength plastic-metal i.e. car body filler. I guess its a similar sort of thing.
PS Another tip: Wax your guide bush with a little bit of candle wax before making a series of cuts and you'll find it all works a lot smoother and it will be easier to make that perfect cut