1. Can I replace these with a single consumer unit? i.e do storage
heaters have to be separated from the rest (because they are on
cheaper rating 7)?

This depends on your Economy 7 switching. Older systems had a cheap rate
outlet that only supplies power at certain times of day. The full time
outlet would always be charged at full rate. Modern systems are the same,
but meter the full time outlet at different rates at different times.

If you have a modern meter and switch, you can combine the consumer units
(assuming you can get one with enough ways) by adding a beefy contactor
controlled by the "economy" output, but powering the things from the full
time outlet.

All this assumes you are on single phase. For 2 or 3 phase systems, it can
get more complicated.

Christian.