What ttype of boiler do you have? Make and name? Is there a hot water cylinder?
Some boilermen like to sell new ones and don't like repairing so much.
Some brands and models are more durable than others.
Winter is the worst time to buy a new one because all the boilermen are rushed off their feet working on systems that were neglected until the weather turned cold.
A local independent, who is on the gas-safe register and is recommended to you by someone you know and trust, may be a better buy than one of the big companies.
If buying, check the boiler makers websites to see what guarantee terms they offer. Often you will get a longer guarantee if you use one of their recommended installers.
If you ask ten boilermen, you will get eleven different opinions of the best brand.
Vaillant, Worcester and Viessmann usually come out well.
Some people grumble that Viessmann parts are expensive and not widely stocked. Since mine has never needed any, that doesn't concern me. I would be more concerned about a brand that was so unreliable that you could buy spare parts on every street corner.