Can you dig the pipes out of the concrete, and then insulate them, or even just run new ones. Putting in a new combi boiler just to supply the hot water, is a bit overkill, and as Ianmcd has suggested, a good cylinder will recover quickly enough, but it'll take up space. If you're thinking of a new boiler, then I'd be inclined to suggest removing the old combi, and siting the new one closer to where you want the water, and then link into the old heating pipe work. Unless you need a high water flow rate, then an unvented cylinder won't give you a lot of benefits, and would require a yearly service. How many people use the hot water, and what are your requirements.