It all depends on how strict your insurer is. Some are pretty lax and will take your word for it that it's restricted, others will insist on sight of a certificate.
My advice would be to have it doen properly, whilst it's expensive it doe mean that you are sticking to the rules and there's no chance of your bike not being restricted, which could invalidate your insurance.
Better safe than sorry, as getting caught (even if a genuine mistake) means you start all over again with the 33bhp 2 year limit.