Just to answer my own question and for the benefit of any future readers, I fitted a 2nd hand ABS unit to my car and it works fine.
I was thinking the information given above by mord might be incorrect - perhaps the ABS units are programmed to the parameters
of a car (in fact that would make sense - it could benefit from knowing the weight distribution, for example) but as far as I know the only parts of a Laguna that are uniquely coded in such a way that they will not work in another vehicle individually are the
, Keycards and Steering Lock.
However, I don't think swapping parts with totally different part numbers is a good idea, unless the different part number is purely because of a revision of the part (many parts listed in Dialogys have several replacement part numbers which have come about over the years since the car was originally specced.)
The ABS controller has no revisions listed for my car, the part number is 77 01 206 689 without ESP - the same part is listed if I search on my 1.8 16v hatch so it's not related to body style or the wheels (and hence brake disc diameter), although there is an alternative part number listed for cars with ESP. A 1.9 or 2.2 dCi around the same age as my car came up with the same part number so I dug a little bit deeper with the specs I was looking up and it appears that it's related to fabrication number - later cars are fitted with a different part number. Since I don't understand how Renault's fabrication numbers tie into year of production and don't have the type (e.g. KG0C) and fabrication number (starts with a letter S then 6 digits) for a car that's not a 1.8 16v made in 2001, I can't establish if there were more than two different parts for cars without ESP, but I'd be tempted when looking for a replacement to go more by model year than anything else. Do the actual parts have the Renault part number on them anywhere, or just the manufacturers'?