This is an easy one.
I had the same problem - drove me nuts, esp when taking a bath.
The hot water tank is in turn fed from the water storage tank - the one with the stop-**** in. The pressure of the hot water is directly affected by the speed at which it's replaced by the cold water tank. What happens is over years of use, the stop-**** gets clogged - there's only a minute hole for the water to feed through anyway. So if you replace your stop-**** assembly, (costs about a tenner) you should solve your problem. I did it a few weeks back, and it's made a huge difference. (Stop-**** assembly just screws on and off, inside the water tank)