What is the name of this company who failed so badly at the 1st hurdle when one of their* engineers, sent to investigate a reported fault/service a newly installed boiler could not be bothered to figure out how to help you out, after they have taken your hard earned cash!
How much effort would it have taken them to speak to you to establish just how comes there is no access to the boiler for servicing?! If it is purely down to a poor installation then the engineer should have been your voice back to the installer telling them what a rubbish job has been done and that they need to put it right!
If the access problem is something that you have caused, again surely they could have worked with you at the time to explain the issue and then with your permission made an attempt to get the required access.