Low hot water pressure (combination hot water tank)

Your fundamental problem is not considering the limitations of your inadequate hot water supply before embarking upon a new bathroom installation. Any plumber / bathroom fitter with half an ounce of decency would have told you that you can't make a silk purse out of a sows ear. Furthermore I am guessing that you've gone from old fashioned taps to modern mono types, which by their very nature restrict the flow even more. Just because they look nice doesn't mean they're suitable. You may also be getting a backflow issue depending on the type of taps now fitted, or indeed may even have a check valve in place on the hot supply to comply with water regs on certain types on installation. My first line of advice to you prior to updating your bathroom would have been to address the issue of an inadequate hot water supply & if your funds would not stretch to that then to postpone the bathroom & upgrade your hot water system first. Keeping in mind your expectations with regards to the equipment you wish to have fitted in your new bathroom, install the most appropriate system to achieve your eventual goals. I am sorry, but you have put the cart before the horse & there is no clever trick to get around this. Don't throw good money after bad with silly idea's like adding an extra ballvalve. It will not increase the pressure or tank volume, probably won't fit anyway, & what's more will take a hefty chunk of what it would cost to put in something decent to do the job properly. The day people stop using have a go handymen, listening to Tommy Walsh & using proper tradesmen instead, is the day that there will be far less posts like this one plastered all across the internet. I hope this helps & serves as advice to others. Get a proper tradesman.

To start with, you say you have a 'combination hot water tank'? I assume you mean a hot water cylinder with a tank built into the top of it? If this is the case, then your plumber is right in that a pump would empty the tank quite quickly, but there is a solution. A single impeller pump, such as a Salamander or similar, mounted beside the hot water cylinder and piped into the hot water outlet of the cylinder will certainly increase the pressure and flow at your hot water outlets (taps, showers etc). To prevent the tank (on top of the cylinder) from running dry, you could add a second inlet valve. I've done this many times in the last 20yrs or so, where a larger tank isn't an option for whatever reason (usually physical size/weight). A second inlet valve would maintain the water level in the tank much better as the pump draws water off. The downside is that you'll also have to replace the overflow pipe from the tank, as the usual sized overflow wouldn't cope if the valves failed. I usually use a 40mm tank connector a couple of inches down from the top of the tank and then run the overflow in 40mm. I usually do this sort of work on a no-work, no-fee basis, to give the customer the peace of mind that if the plan doesn't actually work, then they don't pay anything. If your local plumber is willing to do the same thing, then you're onto a winner! Best of luck, Dave, DrainMedic

Unvented cylinder.

was the pressure ok before bathroom fitted could be high pressure taps on gravity system or high point on pipework causing airlock