Hi Daniel 

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I checked the forklegs were straight first.  You can do this three ways: 1. squint down them and use your judgement! Not very accurate but OK for crappy bikes and it's remarkable how accurate your eye can be!  2. Strip the legs down and use a good straight edge and some feeler guages (better) or 3. if you have the luxury (as I did) strip the legs and set them up on a milling machine in Vee blocks, attach a Dial test indicator to the quill of the machine, zero it on the leg and run it along the leg watching for any variation.  Rotate the leg 90º and do it again.  If it's straight (luckily mine were!) you're ok.  If not (my last bike wasn't) you can either scrap the leg if it's properly paggered (i.e. kinked or really bent) or if you're canny you can straighten it with a hydraulic pipe bender or a PortaPower but it's chancy.
Anyway, assuming all is ok - reassemble everything back onto the bike.  Leave the top yoke clamped up but slacken the bottom yoke off.  Sight the alignment up, and if it's out, grip the wheel between your knees and pull the steering straight.  It's a bit heath robinson but it'll get it somewhere near.  I then aligned the front wheel with the back using a long straight edge, and then took measurements from the bar ends to a fixed point on the frame, such as the corner of the bend (OK on the boxy H model) but you can use any point as long as it's easy to measure and the same on both sides.  If both measurements are the same, nip the bottom yoke up.  If not, adjust, realign and remeasure.  If you're still worried, get a good garage to check the alignment afterwards.
Hope this helps and isn't too blethery!!
phil