I looked into this 3/4 years ago when we were selling our last house. Like you say, the plot is the most difficult bit, and one we never sorted. Even then, land seemed hideously expensive compared to rapidly falling house prices at the time.
If you can, get yourself to a self-build show - I went to one held at the NEC and it was an absolute goldmine of useful stuff. Lots of vendors, obviously, but some useful talks and presentations as well.
Just had a google, it was one of these :
http://www.homebuildingshow.co.uk/We didn't end up doing it, although we did end up buying a house that was self built by the previous owner. I've come to the conclusion that a good (and nicely unique) design is far far more important than all the funky bits you can stick in that a developer normally wouldn't.
How far have you got? I've got a few books still on my shelves that might be useful if you've not got research material yet.
If you're looking for an architect, the husband of our office admin is local, works for himself, is well regarded and a nice bloke.
http://www.johnmorrisarchitects.co.uk/index.html