Poor hot tap flow rate

I need some urgent advice!
I live in a flat and have a new warm air system with a direct combination cylinder for the hot water. Cold water is mains fed - I think all this is correct but I am totally non-technical!
I am having work done by a tradesman and I need your advice to make sure I am not getting taken for a ride as I am not happy with what he is saying.
He installed for me 2 new basins in the bathroom and bedroom and a kitchen sink. All of them have mixer taps which I bought and selected ones for low pressure systems - except the kitchen one which was supplied by my guy and I don't know what spec. that one is. The bath is the original one with seperate taps.
The problem is that the hot water flow is terrible(cold is excellent). The two basins are pretty dreadful and the kitchen one virtally non-existant. The hot bath tap, however, is very good indeed! This is the only appliance which has a. original pipework not touched by my guy and b. single taps. Therefore I am struggling to discover the true cause of the problem.
I have been told by someone else that flexi-hose is very narrow and may cause the problem. this has been used on the sinks my guy has put in. My guy said the only way to sort it out is to put a megaflo system in which will cost a small fortune and it seems silly to do this just for the sake of 3 taps! I guess this will increase pressure of water but not the amount coming out - am I right?
Can anyone suggest what the problem might be or what to do? All I can think now is that my guy has done the wrong piping and may have supplied an unsuitable kitchen tap or I might have to ditch my new sinks and taps and invest in new ones without mixers! That would be very costly too! I can't do anything about the head of water as I live in a flat and there's no room.
Help!!
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It sounds like you have a restriction in the hot supply to the new fixtures.
Since the cold flow is good, it would seem the mixers are not to blame.
Since the hot flow at the tub is good, it would seem the water heater is not to blame.
I assume the pipe size for the hot and cold are the same, so since the cold flow is good, it would seem the size of the hot piping is not to blame.
Since all the new fixtures and only the new fixtures are affected, I would want to look at where the new hot piping is connected to the existing hot pipe from the water heater, to see if the flow may be restricted there.
It may be necessary to disconnect the hot supply at various points to test the water flow directly from hot pipes. In this way the point of restriction may be found.
Mark

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from what you say it seems that your hot water system is gravity fed i.e. it depends upon the height of the tank to give you a greater water flow
in the u.k. bath taps are generally fed by a 22mm. pipe which will give greater flow rates whilst kitchen and bathroom mixers use 15mm. pipework which reduces flow but they sometimes can even have 10 or even 8mm. connections which reduces the volume of water to a trickle
the only sloution to this problem is to either raise the height of the existing tank or fit some kind of mains fed system i.e. combi boiler

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