That is to move the cylinder up into the loft. I realise I will
have to replace the HW cylinder and CW tank with a combination unit

If installing in a loft, install a mains pressure system. You need to check
that your mains flow rate is good enough first, but a mains pressure system
solves the problem of needing lots of cold water storage above the hot water
cylinder. It also gives mains pressure hot water to the taps and superlative
shower performance. They also typically have higher bath filling rates. I
even use mine with a hosepipe to wash the car with a foaming hot brush.

I have a DPS Pandora heatbank in my loft, which works very well and is easy
to install if you are happy doing pipework. You could also install an
unvented cylinder. For advantages and disadvantages of both, google groups
is your friend. The differences have been done to death in the past, but I
suspect the heatbank is likely to be better for you.

Christian.