Ok so I've solved this myself, a bit obvious once you know what is wrong. Essentially the new router has come with a similar but ultimately difference IP address, my old speedtouch router was 192.168.1.1 the new Netgear router is 192.168.0.1. This meant my default gateway had changed as that is the same as your router IP so my NAS was not only trying to talk to the wrong IP address but it's own IP address wasn't part of the range of available IP addresses for the router.
What I had to do was run the Synology DSAssistant software, it found my NAS but couldn't connect as described in my original post, I then selected the NAS and clicked 'Connect' and it took me through a configuration process. You need to know the login details for your Synology NAS to give to the DSAssistant software, then give it your default gateway and give it a new IP address, it seemed to have already predicted the other settings so that was all I needed. Then oked that and it now said connected. I selected 'Connect' again and it took me to the URL log-on screen for the web interface.
You need to note that the URL for the log-on screen has changed in line with your new IP address for the NAS. So my old NAS IP was 192.168.1.124 the new address I told it to be was 192.168.0.124. The URL link to access the NAS has obviously also changed to ...http://192.168.0.124:5000/
Although I could access the NAS via the URL interface the NAS was still not visible to my media streamer or to my other PC's connected to the router and the bitorrent application built into my NAS wasn't connecting or downloading anything.
The answer to the bitorrent and I think to the media streamer was to set-up port forwarding again on my router. I'm not sure which port forwarding worked for the media streamer or whether just sorting out the IP and default gateway sorted that out but I opened up HTTP port 80 to 80, HTTPS 443 to 443, FTP ports 20 to 21. What definitely got the Bitorrent working was opening ports 6881-6889.
What I'm still figuring out is how my other PC's can see the NAS on the network, they're all part of the same workgroup but I think I need to tell my PC's that the default gateway has changed. I can't see the NAS but strangely the windows 7 pc's can see each other's shared folders. When I figure this out I'll update you. Hope this helps others. Let me know.