Only thing you get is £10 of your road tax, on the omega.
Whoop de doo. 
Only if its new enough.
And it costs much more than a few quid in sorting it out, due to the inefficiencies of any Civil Service department. If it wasn't mandatory, I wouldn't bother 
If you visit a local office it's easy 
Complete 'dangle berries'
. Its still a beaurocratic ping-pong between local office and the Welsh tarts.
I had no issues, although they were a but slow issuing the new V5
The local office cannot issue a v5. You have to surrender/buy new tax locally, then send off the guff to the tarts. Then, about 5 weeks later, you have to send all the receipts, which the local office have already verified, off to the tarts.
Wholly ineffective. Too many 'tards to do one job.
I only had to show paperwork at the local office and they exchanged my disc there and then. Swansea were just a little slow issuing the new V5.
Not sure what your issue was TBH

Local office supplied the tax. I lost 1 month, as if I wanted refund on that last month, it would need me to have sent it to Swansea, and being untaxed in meantime. I could live with losing a months tax, to save dealing with thick DVLA retards.
So local office sold me new tax disc, at the LPG rate, but I would have to send V5 off to be updated. Which I did. Again. Only for the thicj DVLA 'tards to initially lose it, then insist on seeing all the same paperwork that the local tax office saw.
All in all, it was about 12 weeks from start of process to getting new V5. 'tards, the rather lot of them.