Do you have a magnetic filter? Magna Clean or Fernox TF1 or any of the the others are a good investment you should only need to do it once so it is clean, unless you are introducing fresh water on a regular basis i.e. having to top up the system water mine is clean and only top up the Sentinal if I have removed a couple of rads for decorating

Yes I have a magna clean which is cleaned every 12 months and I use sentinel replaced every 5 years. I converted to a pressurised system 5 years ago as our loft is very low and I am 6' 2". I put the expansion vessel, etc. next to the heat pump in the 'plant room' . We have partial UFH so I want to minimise sediment, which is why I am probably going over the top! But hey, that's what I am like, and its fun doing it! Of course I tell SWMBO it is essential maintenance, just like the FL2!

One supposes that as you have had a boiler for 20 years and all you have spent is a couple of hundred quid over that time then you have done well. looking at it that way with no maintenance contract, not taking out a maintenance contact has enabled you to save the cost of at least two brand new boilers, probably more. Modern gas boilers need very little maintenance, they are clean burn and the failures tend to be electronics, changeover valves on combi types and washers, some makes have built in obsolescence by having alloy heat exchangers ( you have alloy, brass, copper and a bit of steel, alloy is a good disposable anode) I bought a 28Kw combi condensing boiler 10 years ago, no maintenance contract for £500 I have spent under £300 on spares mostly electronic boards ( the starter ignition board every 5 years) one of which I have repaired as a spare. Basing this on a minimum of £20 per month that is £2000+ per year, that's the cost of two boilers or more from the net or even B&Q. If you are a fairly average mechanic with a bit of electrical knowledge (use a multimeter on all three functions and understand the readings ) then a maintenance contract is throwing money away. my PC is slightly to the right of Genghis
2012 HSE SD4 In Orkney Grey now gone, best car ever.

As a time served, registered gas engineer I would ignore all of the advise on here by people that really have no clue and shouldn't be commenting - except 'gasman' who has clearly been in the trade. Talk to someone who is qualified and knows what they are doing - it's not worth the risks. I hope that your appliance is still working safely following your component replacement.

I hold my domestic gas ,fires ,cookers boilers,water heaters,commercial gas, catering gas ,testing and purging,LPG unvented,so not everyone talks rubbish a lot of what has been said is correct

I stand by my advise - get someone who knows what they are doing out. As someone who has also investigated many Carbon Monoxide incidents as an expert witness, some of which have been caused by people working on gas appliances after taking advise from Internet forums, the results can be devistating. In the works of motorcycle racer Guy Martin "when your dead your dead".

We had the same problem and it turned out to be a faulty electrode ( not sure of the professional name of it ) that gives the spark to ignite the boiler, the gap had opened up to far due to the electrode deforming over 7 years. Replaced it and is now 100% for the last year or so.
Have you thought of taking a maintenance contract out and claiming on that after the initial 14 days no claim clause??