Moi? whysatthen?
Right. N-reg means later type covers. They are a pain in the bottom on a 405 as the bolt "rount the back" on the upper cover is almost impossible to access. You might be lucky and have a spanner *just* the right size, or you may have to take the intercooler and turbo pipes off..... which I would probably reccomend anyway, as to get the upper rear cover off with the turbo feed pipe in place is asking for something to be broken.
The procedure for the belt itself is just like any other XUD. 8mm timing pin behind the starter motor, and 3 M8x50/60 bolts to hold the cam/fuel pump pulleys. Check the water pump very carefully, and if in any doubt whatsoever, change it. Same goes for the tensioner/guide rollers.
Does this model have A/C? If so, the access at that side of the engine is not good, as the A/C compressor location means the PAS pump is up in front of the injection pump. This restricts access to the front covers, and means the engine cannot be moved very far without the PAS pump going through the rad (which is bad

)
Another access problem is getting the chunk of cast iron off from above the upper engine mointing. It is held on with T40 setscrews, which you can't get to because of the brake pipes running to the ABS unit behind the OSF headlamp (which is also a bit in the way for access to getting the aux belt on and off again.)
All in all, the procedure is straightforward, and no 405-specific tools are needed, but access is not good at all. Allow at least 4 hours if this is your first 405 TD cambelt.
Just a thought: Is it looking like the car is going to need a clutch soon too? I only ask as if both a clutch and belt are needed, then it's quite a bit faster to pull the complete engine and box, do the belt and clutch in about 20 mins each, and then refit complete. Don't worry about the A/C being a restriction to this, as the compressor comes off with 4 bolts, and you tie it to the underside of the front crossmember. Nothing else gets in the way.
Hope this is of some help.
David.