Well after 5 days of it failing to download on any WIFI connection I tried, suddenly the other night it said it had downloaded it. Hooray!
So I told it to install it, to which it told me I hadn't got enough space so I removed eveything I'd installed..... still not enough so I ended up uninstalling everything that it would let me on the phone till I was down to the bare minimum of apps and finally it decided to work.
After the update it decided it fancied going into a boot loop just to finish off the joy. After 20 minutes of fiddling with power and volume down combinations I eventually managed to get it to boot up and now, after another 2 updates, it finally says "Your software is up to date"
That was a lot more painful than it needed to be but we got there in the end!