So I guess the next thing is to tell SRCF that you want them to host the website. Once they've set up their end, use the control panel thingy for 123reg to add something that says that www.samandalecswedding.co.uk is an alias for kern.srcf.societies.cam.ac.uk. I don't know what their particular control panel looks like, but for the Gradwell one, I ended up filling in boxes so that I got something which looked like this. The relevant line for this is the bottom one, which says that blog.noctua.org.uk is an alias (or CNAME) for livejournal.com.