The plumber has been and gone within half an hour and we didn't even know each other existed this morning!
Apparently it was just something that was set wrong in the new emersion heater but it was fixed within a couple of screws of his screwdriver and it was sorted! We now have hot water again (or at least we will when the tank has refilled), no more drips from the light socket in the house and an added bonus is that the overflow into the yard won't run any more!
Literally the only negative was the price - £108 but our peace of mind is priceless so it's worth the money and so much more to us!
It's an amazing company that we'll definitely be using again if we need to! If you're in the UK, or more specifically, Gloucester and you need a plumber but you can wait until a weekday, definitely go with Able Group - they are a spectacular company who value their customers... you get what you pay for I suppose!