We run a combi, and run the shower directly off the boiler.If one of us is having a shower, we make sure we don't use any taps as the pressure drops and the shower gets cold.Combi's do this. The upside of a combi is that you aren't heating water and storing it that you don't use. The downside is that they can be very slow to run a bath. Our old combi was like yours, it was a cheap one,I got a better one and it's fine.If your place is a council refurb, it is likely they fitted a cheapy boiler. My thoughts, live with it or fit a better boiler.My last cost about a grand all in, and has been fine.