Any qualified electricians here?

[EddyCurrent]

Things change, I'm qualified but not Part P certified and have always worked in industry rather than installations so I can't say anything for certain. If it was for myself I would see if the local library had a copy of the 17th edition Regs and follow the advice in that. I won't say how things used to be done because it won't apply and will just confuse the situation but I think you could install it yourself as per the latest regulations and have a Part P electrician test and sign it off, I also think you have to seek permission or at least inform the local council about the installation.
6mm2 sounds good to me as does running it in a conduit but I don't think you should run cables of different voltages within that conduit.
At the end of the day it's the 17th editon Regs. and any Part P supplements you need to follow.

Also are you proposing 2 core or 3 core cable ?, if you follow the advice of Web Goblin I think he is suggesting 2 core (Live & Neutral) from the house and providing the 3rd earth core locally via rods.
I also agree that if this is the correct route to follow you should not attempt to install earth rods yourself as installation is not just hammering a rod into the ground, at least it never used to be.

[mekanik]

Can't see a problem with armoured cable for the power supply,then just install low voltage @ internet ect in a seperate conduit.I have no electrical qualifications but i have been looking into this because i have to do something similar.
If anyone knows the separation distance from power cable to ethernet/telephone line that would be appreciated(power & Ethernet in same trench)
Regards
Mike

[m_c]

Fit the biggest you can.

Even though what you've got might only draw 22amps, it's always better to allow for future purchases as best you can. If you only fit the bare minimum, the chances are it'll cost you a lot more in the long term.

In the latest TLC catalogue, there's only £27 difference between 50 metres of 4 and 6mm2 two core armoured, and £30 for three core.