Right, sorry to drag this back up, but I thought it'd be useful to post up what happened in case anyone else is contemplating installing 3 phase.
After my quote from the leccy people (following a site visit to discuss routing etc), I sent them all the plans etc they wanted (just used aerial photos/maps from the net and drew / wrote on them).
Fortunately, the 3 phase line went right along the edge of the property. So I just had to send them the cash, book a date for the installation, and prepare a trench between their line and my garage. Here's Jez (Dollythelw, left), Andy (Moglite, on digger) and a non-forumer having a go on the digger:
Click to view attachmentI threw in some conduit from Jewsons (got a couple of spare lengths if anyone needs?), put a pull-rope through as I went (to pull the cable through the buried conduit) then filled it in. (Somewhere around this time was the digging up of my neighbour's phone line, but lets move on...)
I had an electrician fit everything inside. I had no idea what 3 phase even looked like, so I wasn't gonna risk doing it myself. I now have the normal ring of 240V sockets, some single-phase (blue) 'ceeform' sockets of various amperages, and 3x 3-phase (red) sockets (1x 64 amp, 2x 32 amp). Its not cheap, and I can always add more / bigger if I need to. Its all shielded cable, of course.
Here's the fusey bit, my electrician's work started at this point:
Click to view attachmentHere's some of the sockets. Plugs to match can be seen on the shelf:
Click to view attachmentOnce thats all done and ready, you can get them leccy board to come and throw a cable in, connect one end to the grid, and one end to the consumer unit (which they supply and is part of the quoted price):
Click to view attachmentEt voila. 3 phase 415 volts. Nominally I have a 100 amp supply, but there is nothing limiting this. I could have more if I needed it by just 'telling' the board that I'll be taking more amps. Its just a figure so they can keep tabs on approx maximum demand.
Costs were very roughly 900 quid for the leccy board's work (fitting the cable to the supply, the consumer unit etc etc), and about the same again for the electrician, all the internal cabling and sockets / plugs etc.
Regarding running costs, to have 3 phase rather than single phase adds an incredible £2.21 per quarter to my bill...
Hope that helps someone! Cheers, Al.