they are serviced each year .. the issue is when the service engineer suggests a power flush and you effectively lose your service plan until you pay many hundreds for a a service you may or may not need .. i have never had one engineer suggest i should put a filter in
Did you ask them how sludge could affect ignition/ flame rectification? If they refuse get someone independent to sort the problem and check the water quality.
I went out to someone 4/5 years ago british gas refused to do anything as "there was sludge in the system" so they got worcester out that replaced loads of parts then refused to do anymore until the system was power flushed as it was apparently checked up. I got the phone call and customer told me what the problem was I said it doesnt sound like a sludge issue so I will come out and diagnose. £110 later boiler working perfectly all radiators hot top to bottom.
The sludge thing gets used far too often to get out of insurances so question them about it as there are only a few that I can think of that would cause an ignition lockout