In reply to Swig:
With my Vaillant boiler the warranty says it must be serviced by an approved person.
Once the heating engineer who installed it, an approved installer, decided later there was not enough money to be made on the servicing side, I just went to Vaillant direct and never had a problem.
Costs another £25 per service, but the once I had to call them out when the boiler stopped with a fault, it was next day service by them. They knew which part to bring just having been told the fault number displayed. Their customer service centre been excellent too.
I’m coming to an end of the warranty, but I’m thinking to just stick with them.