I don't know the law in the UK, so I'm speaking from a perspective of the EU citizen. This might however be at least partially relevant, as the OP's van left the factory when the UK was still in the EU.
How do I know anything about it? Looking for the van I came across different configurations: 1+2, 1+1, 1+0 with original mounting points for the seat and the seatbelt, 1+0 without the original mounting points. As all I need is precisely 1+1 I had to check if and what would be possible to modify in a way the van could be registered, insured and used legally. And not for lots of money.
Every car / van has a
homologation. The factory confirms that the vehicle was built and homologated according to a set of norms and issues the
Certificate of Conformity which includes the vehicle VIN. For the EU vehicles (which is the case I assume), there is a homologation number on the identification plate (under the bonnet for the triplets). The vehicle leaves the factory with the
primary homologation + the appropriate CoC. A part of the vehicles is then transformed by specialized companies that can modify the setup and issue a
secondary homologation + CoC, which void the primary ones. The process can be repeated several times (but usually the 2nd one is the last one).
Only the final homologation is valid.
Especially vans are frequently transformed according to the needs of the first customer, and it those transformations are out of the possibilities of the factory, it's done by the builders / converters / upfitters (whatever you want to name it). Some of the modifications are minor, some definitely major.
The 1+0 without the original mounting points I've mentionned earlier is the major modifications case, it included the following changes on the passenger side: seat removal, floor lowering in order to make two or three steps, A-pillar modification, B-pillar removal, door removal, new B-pillar + new pneumatic door installation and lots of minor adaptations. The van was originally homologated for 3 people, but after those modifications homologated and registered for 1 person only.
Even in such case adding seats was theoretically possible, but economically unjustified, as it would cost a fortune, because the van would require to be rebuilt and rehomologated. I've asked for that more or less 4 years ago and only the testing procedure costs
started from like 5000 EUR. In every case if a rehomologation is needed a new homologated seat + seatbelt + professionnal instalation are required.
The general rule is that you can lower the number of seats starting with the last valid homologation (3 to 2, 2 to 1), but it still requires some paperwork (as in your case
@Reg 65). But you cannot add seats to have more than what the homologation + CoC say. Adding seats could be possible only if previously the 3rd seat was removed by the owner (also like in your case) and... there are two cases now: in Poland it would work only if the removal was not mentionned in the registration documents, but in Slovakia that could be done if the homologation / CoC were not changed. I found companies that are buying the van from you, sell it to Slovakia, make the modifications and the paperwork, then register the van there with the new setup (for example 3 seats) and sell it back to you. Which gets problematic if you sell / buy the van as a particular, not a company, because there's VAT to be paid in some cases... And those companies are not doing that for nothing.
I know and understand that the factory mounting points are the same for the single and double paseenger seat in the triplets (probably also all the other siblings now) and only the external passenger seatbelt it attached to the van body, but this only facilitates the process if you want to go through that in the proper way (modification by the professionnal + rehomologation).
Note that the seatbelts are different with / without the passenger(s) airbag(s) as the clip you attach the seatbelt to is different with a pretensionner than without it.
I will also say that I personnaly find this law was necessary as there were many extremely cheap modifications made with no respect to physics and rules which caused several casualties during accidents.
And there are still people that modify their vehicles without any self-preservation instinct. Earlier this year I've found a youtube video of a guy that attached the passenger seat with a construction grade long threaded rod instead of a high tensile factory bolts (because the bolts are deep in the tubes that hold the seat in place, so are difficult to reach). And found nothing wrong with that... The factory bolts are like 10 to 20 EUR each and the appropriate socket and longer adapter to reach the bolt wouldn't probably exceed 5 EUR. Pity the guy haven't changed the bolts to rods on his seat, but on the passenger one.
As this subject is complicated and might vary depending on model / year and country, such modifications should be discussed with the authorities before any investments are made.
The van with the pneumatic door was the second most ideal for my needs from what I was able to find during almost 2 years. I was very close to buying it, but luckily I've checked it earlier and found out that I wouldn't be able to use it for my needs.
EDIT: FYI, the van I finally bought was fifth most ideal only, but became the first one as the others had such (or similar) issues as described above 
