Actually my current setup is something of a dogs dinner:
There are no tanks in the house anywhere.I have a boiler in the kitchen that provides mains fed hot water throughout the house (is it called instantaneous?) and another in the garage that provides for the central heating. The central heating one is open vented fed from the loft by a header tank. Both are ageing and not condenser types so I imagined changing the central heating boiler for a combi to serve both hot water on demand (like the kitchen one) and run the central heating as well. Then I can discard the kitchen one - freeing up space. Am I making sense??
I have a scrappage voucher with a few weeks left on it so I wanted to take advantage and upgrade to the one boiler - preferably keeping the flow rate of hot water quite high (hence the original subject line of this thread).
Thanks again everyone - really appreciate this good advice
Ahh, now it makes sense!
The only problem with a combi is that if you're in the shower and someone turns a hot tap on in the kitchen, you'll know about it!!
My advice would be to scrap the water heater in the kitchen and get a new system boiler in the garage, along with an unvented water heater.
This set-up will give you a large volume of stored hot water that will be delivered at mains pressure to several taps at once.
The other option is a storage combi (I think Vaillant do them) which is, as the name suggests, a combi boiler with a small storage vessel built in which goes a long way to overcoming the dips in hot water flow that plague combi installations and gives a decent continuous flow of water.