WHAT CAUSES THE PRESSURE ON MY COMBI BOILER TOFALL TO RED. i HAVE TO KEEP TOPPNG IT UP TO ONE TWO BAR?

You have a leak. This is the only answer, check all visible joints and rads. Odds are the leak is under th floor or inside the boiler. Is prv constantately dripping.

Primary reason for the pressure failing is a leak somewhere. However, you are topping the pressure too high to start with. 1bar to 1.5bar is preferred.

I'd check the 'overflow' pipe outside for signs of discharge. This is your pressure relief pipe, and if the pressure exceeds 3bar it will discharge. When your heating is on, make sure the pressure doesn't start going right up, if it does you need someone out to check your expansion vessel. As it goes up when heating is on, discharges, then drops when cooled, you see low pressure, add more water and cycle continues.

Check around radiators for leaks, as any leak will cause the pressure to drop.