If anything happens at that property the landlord is legally accountable. I'm sorry, but if he has been employing engineers who can fit a boiler, with a vertical flue, plus 2 rads in half a day, they are obviously cutting corners! Impossible to complete that work to standards! I would not re-attend until your money was paid... Not a chance. He is taking the p***! Tell him you will not be fixing any leaks until your initial quote/invoice has been paid. He knew the cost from your quote prior to works being carried out, Very frustrating! You are trying to complete gas works to a safe standard and this guy is basically willing to put his tenants lives at risk expecting the job to be done in 4-5 hours! Or... you advise him you want paying up front to re-attend... but then you run the risk of not being paid should the job be more than just a standard call out. I agree with matchless.plumb... You need to seek advice and this guy may need to be reported... he may be a member of the NLA or the RLA. I hope this works out for you I really do.