Hi,
How long have you owned the bike, is it registered to you? SORN applies to vehicles whose last tax disc expired after 31st January 1998. If it has been yours (registered to you) all the time it has been in storage then you may have to bite the bullet and cough to the DVLA. Try speaking to someone at your local licencing office, off the record and see if something can be sorted out. If you have never registered it, then do so now and then SORN it like you've just bought it.
I bought a bike over a year ago and did nothing until I'd got it ready for the road. I registered it eventually and put that day's date on it. I then sent off to sorn it to be informed that it had been off the road for 14 years and didn't need a SORN until the next tax runs out.