Minimum head of water?

You don't mention what available room you do have, but consider that a combination cylinder has it's header tank sat *immediately* over the hot tank.

It's not the head of water as such you need to worry about, (as another poster has said) you can place the header tank immediately above the HW tank, but if you want to have a hot water feed above the level of the header tank you have either to forget it or use a pump of some sort !

As others have said, it depends where the taps are which are being supplied by the cylinder. The "effective" head is the vertical distance between the tap and the water level in the header tank. Ideally this should be a couple of metres. The cylinders which have integral header tanks are only really any use if they are only supplying water to the floor below. If you have a first floor bathroom and a first floor cylinder, you really need the header tank to be in the attic.

If you cannot do this, have you considered an unvented (i.e. mains pressure) hot water system, which doesn't need a header tank?

If it is for a shower then you require 1 metre from base of cold tank to shower head ,as per previous post combination tanks have head of water on top of hot.

If space at a minium check out a non vented cylinder works direct from water mains,much more expensive.

Consider using a mains pressure system, such as an unvented cylinder or heatbank. They are more expensive, but vastly superior to any low head gravity system, provided you have good mains pressure and flow.

If you have poor mains flow, or are short of cash, consider a combination cylinder which has the cold tank sat on top of it in one unit.

Christian.