Is it safe to connect a single socket onto the power point for the old
storage heater,

Whilst electrically fine, there are 2 issues.

1. You must relabel the circuit.

2. It will probably only work during Economy 7 periods. This can be fixed by
moving the MCB and cable to the full time consumer unit from the Economy 7
one (or from after the Economy 7 contactor). If you don't expect the run
particularly heavy loads through it (i.e. another heater or a washing
machine) then you can just connect it to a suitable ring main/socket radial
at the consumer unit and save yourself wasting a way in the consumer unit.

Christian.