I phoned the first dealer again today - it's where I bought the van - and they confirmed what jj9 says. Then I fell down a dealership rabbithole.
As the first dealer's quote of £397 for the 2-yr service was so wildly different to the second dealer's £249, I asked for a quote again, and this time got £249 as well. I queried where the £397 figure had come from and they didn't know, but guessed someone had read a wrong line or muddled which vehicle I had. They said most Berlingos only required a basic oil/filter change to keep the warranty valid, but also said Citroen apparently recommend that doing more than this is beneficial (why not make it part of the schedule then?). The guy then added that "a few engines" did require a "full service every year". I queried whether this applied to me and he took my reg number and then said that actually my vehicle does require a full service - i.e. air, fuel and pollen filter as well as oil - every year. So actually the quote is now £367.
Then I phoned the second dealer again, to ask whether their £249 price was for a "full service". They said that quote was an error because it was for a car and I've got a Berlingo (eh?), and actually the quote should include a brake fluid change and would be £309. After politely expressing my confusion I was handed over to their service manager, who took my reg no. and looked up what Citroen recommends (weren't they doing that before?), which is apparently that my vehicle 's first service should be after 2 years or 25,000 miles (not one year or 10,000 as I was told last year), and would cost £408.85. She said that only the MPVs need a service every year, unlike the van (don't they share the same mechanicals then?). I didn't ask why this info contradicted what I'd been told by the first dealer, nor why the first dealer had contacted me after one-year's ownership to tell me my van needed a service.
So, I now know that the service interval should be either every year or every two years, that Citroen specify a minimum servicing for warranty maintenance but also recommend doing more, and that depending on who I ask the price of the 2-yr service could be £397, £249, £367, £309 or £408.85.
Lol.
I think I'm going to write to Citroen and ask for clarification. In the meantime, I've cancelled my booking.