surely this is just a case of fitting some henley blocks?
You really seem to be struggling to accept that you aren't capable of this work.
To make absolutely clear, it is
NOT simply a case of fitting henley blocks and splitting the supply off down whatever cable you feel like. Even if you do notify this to BCO (which I doubt you intend to do), it seems unlikely they will pass your work, as you haven't listened to a single bit of advice provided so far.
Thanks for your responses everyone.
In regards to a heat pump, i'll forget that , unless i win the lottery this week!
Why? An air source heat pump (a-la air conditioner) can be had for £250 or so off eBay these days. I installed a couple in my old house, one ran 24/7 for a year or so before I moved out and never had a single problem.
As to a lower kilowatt water heater i'll look into that (the 9kw i had in mind is provides instant hot water).
Unless you get a new supply from the DNO, you can pretty much forget instantaneous water heaters. All but the smallest (the type you occasionally see above sinks that provide just enough water to wash your hands) will be unsuitable for your existing supply. Take the advice that has been provided and consider an unvented tank with immersion heater. Oh, and although I'm not a plumber, I believe I'm right in saying that fitting an unvented system is NOT a DIY job. If your granny isn't killed by fire or electric shock first then she certainly doesn't need an exploding hot water tank.
The general concensus seems to be to run the cable from my existing meter and not my house CU which seems a good idea so does anyone know what cable size i would require for this (distance 35 metres, potential load 13kw) ?
That's funny, I thought the general consensus was that you should completely rethink the electrical loads you're considering and then get a good electrician in to design and install for you. Despite pretty much everyone telling you otherwise, you're STILL asking what cable to use for a 13kW load! Nobody is going to answer your question, as you're clearly a danger to yourself and others.