I thought I'd share an update on this to hopefully help any other users trying to do the same thing. The whole process was incredibly frustrating...
I booted the new iMac using Cmd + R, erased the drive (as it still had the previous owner's details on it) and reinstalled Yosemite from new. At the end I selected the option to restore from my Time Machine backup, held on my Air Port Time Capsule.
I started this and the estimated time wobbled around a bit but settled at 10 hours. Not ideal, but given that it was over WiFi I thought I could live with it. Went to bed and left it. Woke up and checked it and the estimated time had increased to 1,240 hours although it appeared that around 20% progress had been made. This made me suspect that it had stalled but I left it alone nevertheless and went to work. Upon returning in the evening it had reduced to 650 hours remaining(!) but didn't appear to have made any significant new progress. I thought I'd leave it another night and in the morning it had 'reduced' to 550 hours. I lost patience at this point, switched it off and went to work.
Upon returning in the evening I repeated my earlier erase and reinstall, and instead of migrating over WiFi chose instead to migrate via a wired USB disk that I'd also backed up to using Time Machine. Much better, visible progress was being made but, rather alarmingly, no estimated time was given. Instead, it progressed to 'Transferring your files' with a completed status bar. I left it overnight once again....
... to be greeted by an identical screen in the morning. I left it running and went to work, and hit the Internet to search for similar issues. At this point perhaps I shouldn't have been surprised to find that this appears to be a very common issue. Buried in what appeared to be a number of very complex workarounds for a novice Mac user were a reasonable number of users who had just left it running, with the process completing after a couple of days(!) at most. Given that there appeared to still be some disk activity going on I left it... for two days.
No progress, and worse the USB drive status light was now flashing, essentially showing that it was sleeping. I hit Cmd + R, skipped the warnings that Migration Assistant may not be complete, and hey presto, ended up in a completely transferred environment. There were a couple of things that hadn't transferred across - iCloud and Dropbox passwords - but otherwise it had completely perfectly.
So there you go, how to migrate 490 gb of data from one iMac to another iMac, taking the best part of a week to do it. I'm not sure that I'll be using this process for the next upgrade which, given the time that this one has taken, is a little closer than it should have been!
