Thanks for all your help. Having done the mod yourself, if you feel that 15w would be the best option for a comfortable, controlled, firm but not harsh ride I am going to go with to start with that and 2 turns of preload and adjust from there if I need to. I know the chances are that it's not going to be spot on first time but I need to start somewhere. I'm around 12½ stone and was a little worried that the spring rate would be too firm for my weight but they seem about right from what you've told me.
How did you hold your front end up? Is it ok to support it on the exhaust with blocks of wood or is it likely to damage it?
Thanks again.