Plumbing advice. Water tank

[Lethagized]

I've purchased an immersion heater cylinder tank which has a seperate smaller tank on top to hold the cold water. I'm planning on fitting it up this weekend, but i've been told i need an extra cold water tank above this tank to increase the pressure otherwise the cold mains water from the mixer taps will push the hot water back up meaning no hot water at the taps. Is this true? Do i need an extra large cold water tank, and does it have to be mounted above the small cold water tank attached to the immersion tank?

 

[Lethagized]

Thanks for the replies so far. I forgot to mention that this will be a fresh installation concerning the cylinder. The pipes are already installed but never connected up. We don't have central heating or a combi boiler.

The combination cylinder will only be supplying...
The bathroom beday which has mixer taps and the same thing with the bath.

The combination cylinder is in the loft, and the bottom of the cylinder is about 3 meters above the level of the bathroom floor.

Another question is, if you had 2 tanks in the loft. One tank holds 50litres of water, the other tank holds 1 litre of water. If they were both mounted at the same height, would the pressure of the water coming out the pipe be equal? If yes then my original question is already answered.

I've also found out that i will need to connect the mixer taps to a gravity fed cold supply because my taps mix the hot&cold in the block instead of the head of the spout. I found this out some time ago when i connected them up to the mains and water was backfed up the hot pipe. I need equal pressure at the mixer taps. Damn crappy B&Q taps. Cost me over £160 for them as well!!