After getting my bike (almost) sorted I noticed that my forks are bent - the wheel is not 'vertical' in them as they are bent very slightly to one side. This is the result (I assume) of a rather embarrassing accident 12 years ago when I rode into the back of a stationary milk float. My front wheel stuck under the back of the float and me and the rest of the bike cartwheeled out into the road. Fortunately I wasn't too hurt, but it did knock my confidence and I think is partly the reason I packed in for 12 years. The funny thing is that after the accident I couldn't ride at all without my hands on the handlebars, whereas before I used to be fine riding miles 'non-handed' as it were. I thought it was purely psychological but my discovery of the bent forks seems to explain it now....

Anyway, back to the point. I need to replace the forks, but I'm confused about forks/steerer/headset compatibility and sizing . My bike is equipped with a 1" steerer that is 20.7 cm from the bottom to the top of the steerer. The steerer is threaded and has a slot in the top at the back. My questions are:

1) What do I need to know to get a new pair of forks - is steerer diameter and steerer length all I need to know, and is the steerer measured from the top of the upside down 'U' shape at the top of the fork arms to its very top?
2) What headset considerations do I need to think about - I've noted that steerers can be threaded or unthreaded - are they interchangeable with the right parts, or totally incompatible?
3) Is it possible and/or necessary to change the headset, or would I need to have the bearing cups removed from the frame, ie could I use the rest of the bits from a new headset with the old cups still in the frame?
4) Can a steerer be cut down if I can find one that is too long, as long as the threads come far enough down the tube?
5) Anyone know where I can get a cheap set of decent forks in the UK? They don't appear to be too common - it's all expensive carbon stuff!