Hi,
>
> I've got a 405 estate (1996 model) and am looking at making a couple
> of changes, partly because my family of four is about to have a new > addition.
>
> First change is I'm wanting to do away with the middle lap seat-belt
> at the back, and put in a standard shoulder-fitting one. I don't know
> if this is even a possibilty, it's just I'm not sure how safe the > lap-belts are.
That's probably pretty difficult. I've only been in one car with a central 3 point belt, and that was a Citroen Picasso, IIRC it was attached to the roof somewhere. Finding somewhere strong to mount your 3rd point is going to be the main problem, and unless you find somewhere pretty rigid it might be less safe than a lap belt if you roll the car or otherwise really bend it.
Secondly, I'm looking at putting in some additional (rear-facing)
> seats at the back (ie. in the boot space) for the children.
The boot space is a crumple zone. You may know what happens to a crumple zone when something hits it. Putting the kids there isn't a good idea.
Does anyone have any experience of doing something similar, and know
> where to go to have it done, and approximately how much the changes
> might cost? I'm a little averse to going to the local Peugeot dealer,
> as I'm sure their charges would be high, even if they did take on such > modifications.
Get something that already has 7 seats and check that the back two are still protected by crumple zones.