We just had a bathroom and ensuite installed on the first floor (where there was previously a toilet and bathroom. The piping has been done with copper pipes but when I am in the main bathroom using the shower (with hot water) and if someone else is in the ensuite showering (the water goes cold in ensuite) and when the main bathroom shower is off (the water ensuite shower then goes hot). We have a combi boiler.

Please can you tell me the reason for this problem?

Since this is a new install you should call back the installer. It is likely the hot supply line is too small and/or tapped fron a main trunk in the wrong location. This could cause a low volume situation and result in cold rather than hot water