You need to make sure that your new shower is in fact suitable for use with a combi boiler - gravity fed system showers simply won't do. The water pressure to the house also needs to be high enough - but I guess if you can shower with cold water or hot water but not in between then it should be. I would have thought that 1 bar mains pressure was a bit on the low side for a 5 storey block. I wonder how you measured it? Not by looking at the boiler pressure gauge though - thats for the central heating water circulation which is pumped.
John
April 2008