The base of the roof header tank is approx 600mm above the shower head,
the minimum head recommended for the thermostatic unit is 0.1 bar which
I assume is approx 1000mm.

Whilst the valve manufacturer claims 0.1 bar is acceptable, most people find
around 1 bar to be ideal. Much less than 1 bar and it will not be
satisfying, unless you are using a low pressure drencher head that doesn't
particularly benefit from pressure. Over 3 bar and the massage can be too
powerful.

I cannot seem to find a shower giving less than 1.3 bar, my main concerns

1.3 bar will provide an excellent shower and needs no modification.

Any thoughts and/or recommendations as to which pump I should buy.

Stuart Turner Showermate 1.4 bar twin.

http://www.plumbworld.co.uk/750-299?...280059f8e95000

If you want to go all out for reliability and unlimited duty cycle, then I'd
go for a

Stuart Turner Monsoon 1.0 bar twin

http://www.plumbworld.co.uk/750-14600

You should buy a pump with negative head capability, as you may find the
flow switches are unreliable with such a low head. A negative head unit has
a separate pull cord to kick start the pump.

However, there are alternatives to using this. If you also feed the bath
taps, then these, being lower down and larger bore will easily start the
pump. I used to do this on my last house. You just set the shower and blip
the bath hot tap.

For a bit more money you can get special flow switches that work
automatically.

Christian.