I know you wanted the sellers to get the overdue boiler service done but honestly, for £100 or so I’d really recommend you get it done yourself and swallow the cost. If there’s a problem with it subsequently, you have a record of what was done, when it was done, and hopefully - this is priceless - a boiler man you can call when you need him, as an existing client (and good payer), rather than wasting time in freezing winter trying to get someone you’ve never met to come and look at your heating when it isn’t working.]