I can't think why they'd be invited to meet the Swedish PM (he was the one later assassinated) just because of ambulance driving - you'd normally expect either some link to Sweden, or perhaps as Chris suggests to the Labour party.
I notice the date you give isn't the same as for the photos Chris links to, so maybe there were two receptions?
You could try the Times archives - they might explain what the purpose of the reception was - or perhaps those of your local paper in the library; they might have mentioned it at the time.