Thanks for all your replies...... I've looked at the tank plumbing and
now get why you can't feed it from the mains.
I still want mains at the taps though, I'll have a dig around and see
how the pipes are run, hopefully they'll be separate outputs from the
CWT going to the tank/shower (has pump) and the rest.
I'll also be careful about mixer taps and potential back flow to the
HWT, we don't tend to use hot water anyway at any sink apart from the
kitchen and that already has mains on the cold.