Hi hopalong,
I could give you more info if i knew what type of boiler you are having fitted ie system, combi, floor standing, wall mounted or and wether it is gas or oil fired ?
But in general these rules apply to most modern condensing boilers. The appliance is a room sealed, fan-flued boiler, consequently it does not require a permanent air vent for combustion air supply and may be installed in any room regardless if it is on an outside wall or not..
As for the flue length you can go upto 4.5m or if you prefer aprox 14ft 9in. In the horizontal position. 5.25m ( 17ft 4 ) if it is installed vertical & using a roof terminal. But you must remember that every 45degree bend is classed as 0.5m of flue length & every 90 degree bend is classed as 1.0m of flue. If longer flue lenghts are required then you can use a Twin Flue assembly and can then flue upto 14m ( 46ft ) but you must class each bend as 1.0m (3ft 3) regardles if it is a 45 or 90. The other factors are the flue termination which is another host of rules and regs but i will give you more info on that if you require it . Hope i have answered your question and best of luck. Gasman.