Steve,
My 'Toast rack' project had previously been of the road for over 16 years & never sorned.
I tried to sorn it when I got it, but was told It couldn't be sorned until it had been taxed, as it had never been taxed since sorn began.... Hope that makes sense.
When I came to tax it, the DVLA system would not let me do it on line, so I downloaded the form (V10 I think) filled it in & took it to the post office with my V5 Mot test cert & insurance cert. I was told I needn't have bothered filling in the form.
I'd suggest you just 'rock up' with your doc's & dosh and the job'll be a good un.