If the bike hasn't been registered/taxed for some time, you may discover the registration number has lapsed. Best thing you can do is contact the DVLA with all the bikes info and ask them how you can register it. I do believe they will allow you to keep an old registration book, best ask before sending it - they lose stuff. You can always claim you've only got a photocopy of the original logbook.
You can phone them on a 0300 freephone number, but I'd recommend you email them. That way you'll have a printable reply, as against being bombarded with info that you'll forget as soon as you put the phone down. There's a list of email contacts for vehicles here
I'd use the last option
(Phone 0300 790 6802 - open 8am to 7pm ish)