What are the rules re phase separation of the different phases on a 3
phase supply? Something like 6ft comes to mind from yonks ago, but
more precise info eludes me & where is it in the iee regs?

There is a 3 phase supply here (prev used for heating, now only 1
phase used for the main house supply) and I'm wondering whether one of
the unused phases could be used for the workshop/garage. One specific
question is how near may 13amp sockets in the house be to ones in the
workshop - especially if extension leads are used.