Sorry to hear of the issue.
If a tradesperson such as a plumber causes damage to your property - you can ask them to return to fix it at their cost. If they refuse, you would need a quote for repairs (if the boiler is repairable) - if not repairable then you would need a quote for a new boiler so that you know how much to pursue them for.
The service they carry out is to supposed to be done with "reasonable care and skill", as per the Consumer Rights Act 2015. If they were negligent, you can sue them for the cost of repairs or if they have damaged it beyond repair, a new (like for like) boiler.
You would need to contact them and ask them what they propose to do about it. If they refuse to help then obtain a quote, then send that to the plumber with a demand to pay within 14 days, and tell them if they refuse you will report them to Trading Standards and you will pursue a small claims case against them.
If you have to pursue a claim, you can do it online - here is the site : https://www.moneyclaim.gov.uk/web/mcol/welcome but you do need to give them a 14 day letter to give them chance to avoid court action.
You would not need a lawyer for this either - a small claims case is designed for people without a lawyer but do feel free to come back to this site if you need any help with it.