i have had a loft convertion built with onsuit bathroom the council said i have to have air test on pipe work

cerbhunt

This is probably because you had a water tank in your loft that you have now removed, it is therefore likely that you have converted to a pressurised water system (you're probably getting the best showers you've ever had in the house). As your plumbing system was designed to supply water from a gravity system (ie from you old tank) the council want to make sure that this system can withstand the, much greater, pressures delivered by the mains water system

speedball1

In a air pressure test the water system's capped off and air is injected into the closed system. In my area it's 125 PSI for 8 hours. If the system holds you're good to go. If not then repairs are in order. good luck, Tom