Really doing me in lately and getting worse.
After several miles my hands go all numb, it's not how the bike is set up as it happened on my mates bike and his setup is nothing like mine.
I really doubt what you say is literally true - and you have to get very literal when it comes to ergonomics! For a start, are both bikes flat bars? Have you considered bar width? Or more importantly wrist angle, where a shorter but closer bar can have the same angle as a wider one further out?
The classic cause of numbness in the hands is carpal tunnel compression, which is caused by wrist angle - if your wrists are at angle, especially but not only if you are carrying weight on them, that's bad. You should be able to find the info you need by googling with these keywords; if not PM me if you want help. Bars like North Roads have a wrist angle that compresses the carpal tunnel less - although you may get by with Ergons and a higher stem. But North Roads, and some dirt drops, let your ride with perfectly aligned wrists, so no problem.