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.