Thanks guys, it was looking a bit iffy for a moment but its now sorted.
To apply on-line the registered address needs to be correct but I didn't know if it had been updated yet or not as I hadn't received the log book, and I didn't have the V11 reminder and very likely wouldn't get it in time by forwarded mail if at all... so that one was ruled out.
To apply at a DVLA office I needed a completed V11 reminder which I don't have, along with a printout of insurance and MOT test certificate (which I do have) so couldn't do it that way either.
I ended up calling customer services on 0300 7906802 and going through option 3 then 2, then navigating several other options I've forgotten
They were able to confirm the address had already been updated and a new log book had been sent out on Tuesday, and take payment over the phone to get the tax disc posted to the new address. Result!
So anyone getting stuck in a similar predicament with lost documents or out of date address just ring that number - all I needed to tell over the phone was the registration plate, confirm the make and model of vehicle and the registered keeper address.
By the way in drivers license news, I used the on-line application (a couple of weeks ago now but forgot to reply to the thread) and got a new license in two days without sending the old one back! I do still have to get the old one sent back which I will do as soon as I get a chance. So doing that on-line seems to be a good way to avoid not having a physical copy, even though technically you can drive without having it on you...(lots of other stuff uses drivers license for ID though so its inconvenient to be without it)
One rather bizarre thing happened with my updated license though, apparently I now also have a class A license as of 19th January 2013 (when some new regulations came in ?) which means I can ride motorbikes ???!!!
I didn't have that on my previous license, and I did NOT sit a motorbike license in New Zealand either - in fact I've never ridden a motorbike on the road in my life!!
In NZ a motorbike license is most definitely a completely separate test and class to a car. They've also given me a class AM license (mopeds ?) as of 12 June 2013 which is when I exchanged my license. I can't remember for sure but I don't think I was allowed to ride mopeds in NZ either...