ferrolli f24 combi boiler with low water pressure for hot water?

I have the above boiler which never really worked well when we had a shower head running off our bath taps but we were told that this type of boiler would be great if we had a thermostatic shower by the fitters and also by a local repair company that we used when we had some problems with it before. We have now refitted our bathroom with a hidden valve thermostatic shower and mixer bath taps. We now find that it takes over 30 mins to run a hot bath deep enough to cover your legs when sitting down (normal size bath!) and that if we run a shower with any power applied to it, it is cold! If you turn on the shower at the lowest possible pressure you can get a warm shower, but this is not ideal! It appears that there is high pressure behind the cold water but not the hot so when both are being applied via bath mixers or shower you end up with cold output! Can anyone provide a simple explanation/solution to this, or anyone had same problem?