Surely you should have done more than took the sellers word that the boiler was ok if it concerned you that much.
It is not (yet) a requirement that appliances such as boilers are given the all clear before a house sale can proceed, and i would assume your solicitor will say that no infomation was put forward regarding the state of the boiler or central heating, (unless it was specifically mentioned in the survey report) therefore they will not take responsibility for any issues, and any further advice would probably be additional and therefore a charge would be applied.
There is nothing stopping a prospective buyer having a specific survey on items like this, and it goes down to the old saying of 'buyer beware'