Please ignore my original post. The problem lies with my 3g cellular network (GiffGaff) here in the UK. Other users on the O2 / GiffGaff network (GiffGaff is an MVNO and uses the O2 network) have reported the same problem with Slingboxes. It would seem that O2 / GiffGaff do something on their network which prevents users from connecting to the slingbox - possibly the required port could be blocked at network level. It has been reported that users on the Vodafone network do not encounter this problem so I may switch to Vodafone. Connecting to my slingbox using wifi on a remote network works as expected, I only wish I tried this first before spending endless hours trying to track down a problem which I had no hope of resolving.
More info on the O2 / GiffGafff > Slingbox problem -
Another post on Slingbox.com forums about this problem -
I think on older Slingboxes there was a workaround whereby if you changed the Slingbox port and forwarded that port on your router it would get around the O2 / GiffGaff network block. Unfortunately the port cannot be changed on the newer 350 and 500 Slingboxes so the only option is to change to a different cellular network provider (forum posts suggest that Vodafone and 3 will work with Slingbox) or accept that you'll only to be able to connect to your Slingbox using WiFi.