I'm after a cpu water cooling, something like the Corsair H90 or the Antec H20 920, however I'm also looking to upgrade to a full water cooled system at a later date.
I want a modular system, so that I may extend it later, however the Corsair is a sealed unit, although I'm not sure about the Antec.
I can find suitable radiators and pipe/pipe attachements ok, but using a pump and tank will not fit. I have room for 1 slot in a 5 1/4 drive bay, I have seen drive bay reservoirs but a suitable option would be one which combine a pump and reservoir in one.
Alternatively, are there any processor block/pump combinations?
Yes, there are drive bay reservoirs with integrated pumps (dual 5.25" bay ones are far more common though) and waterblocks with pumps attached. Reservoirs are unnecessary but can make it a lot easier to fill/purge/make sure there is no air in the system.
What you describe is very similar to the Swiftech H220, which bridges the gap between All-in-One and custom water-cooling by giving you the ability to expand on the loop later on. Though IMO, for the price of the H220 and its intended use, you would be better off going custom water from the start and getting yourself a custom kit.
In water-cooling, near every component is combined with some other component. There are even radiator/pump/res combo's if you really want it.