thank you all for your prompt replies! in answer to a couple of you
the electric circuits are completely seperate making homeplug
inoperable. I would like if possible to keep a physical data link as
ive had bad experiences with wireless reliability, plus using cat5 i
can go up to gigabit if needs be.

As with the comments above everyone seems to have conflicting views on
whether its a good idea to link buildings. Im not interested in using
fibre as its WAY too expensive for this project. A cheapo £20 switch
will be at either end of the line so im not too bothered if the risk
of it blowing is there, as long as it isnt a common occurance!

I'm torn as i would like to try it but dont want to do anything
dangerous in any way. The building is being done up and will have an
office in the end of it that requires internet access as well as
reasonable transfer speed for backup purposes.

thank you for your comments
Pete