my buderus combi needs topping up every 15 mins. I've bled the rads?

if your loosing pressure its very likley your loosing water.
check your radiators for leaks and all your pipework.
check your pressure relief pipe which normally discharges outside. if that is leaking you will need a pressure relief valve.
you will need to make sure your boiler is not over pressurising (3 bar on gauge)
if you have experienced a loss of pressure a few times and then increased pressure (excessive) your expansion vessel is likley to be faulty .

tech boiler svs
February 2009