thanks for your replies,
i have BT home hub version 3 Type B, so as described above this should work. although im still experiencing problems. the Raspberry pi has been setup with a fixed IP. i have setup a device on my router and forwarded the correct port and linked the two together. i have also placed my raspberry pi in the DMZ on my router.
I have tested the port by using http://www.yougetsignal.com/tools/open-ports/
and entering my external IP address and the port i forwarded and it comes back open.
but for some reason when i ry to view my raspberry pi using a camera viewer app on my phone it doesnt work. this will work though when im connected to my internal network and view it this way. I cant view it on a pc either externally even though the port is apparently open.
any advice on anything else i can test? as far as i can tell it should work so my only other option is to wipe my SD card and start again i think.