A quick update:
So, I decided to download all the apps that I wanted from Ovi by using the apt-get install -d command, and managed to get all the ones i needed onto the 'broken' n900 and then bluetoothed them across to the other one (found a sneaky way of finding the install filename)..

Then I found this on another forum from 4 years ago!

"I too had this problem where my 3G connection would work fine with MFE and OVI store but the WLAN connection would fail. The problem was the DNS settings on the WIFI connection advanced settings. I needed to set these to public DNS servers rather than depending on them being auto provided by my router"

Just tried to download via 3g and it worked perfectly... So, there's the answer! I hope this helps someone else..

Thanks all for your help and suggestions, i know the ovi store is rubbish but i hate being beaten, and at least i have a backup of all the apps I wanted now :-D