Well, there is space for a small header tank, but I don't really want
to add another component to my system.

The flow from the roof tank is actually quite good as the pipework is
oversized. I guess if a pump isn't feasible then I will just use 22mm
pipework all the way to the shower mixer. I don't want to fork out for
a high pressure pump and mixer block before I find out I can't use it!