Hairy Monsta.
Have a look at
www.geocities.com/reliantman2002 and go to the tube bending page.
I also have a Machine Mart Clarke bender, which as you said is not brilliant, but has done the job.
I am no expert, and made a lot of mistakes the first few times, but like anything else in life all it takes is practice.
Decide which end you are going to start with, either front or back, obviously, and make sure you take all your measurements from that end.
If you are using chalk or tipex, measure off where you want your first bend, mark it off and then measure to the next bend you want, marking the centre of the radius with the tipex/chalk/ whatever.
If you stick to the same measurements both side you should be ok.
The main thing to remember is which side you are working on and not to make up two right hand lower rails like I did.
Cheers.Doug