What I do know is that I had a letter from Southern electric last year / 18 months ago stating that they would be replacing the teleswitch - is this bodge down to them as I would expect them to install correctly,
Very unlikely - if they were just replacing the switch or meter etc., that is exactly what would happen - old one removed, new one installed in the same place. Meter changing people generally don't have the time to do other things, and even if they did, are probably not trained or allowed to do so anyway.
Do you only have 2 storage heaters? If so, it is questionable whether E7 is actually of any benefit at all, since you will be paying more for the daytime units than on a single rate tariff.