OK , boring post from me, but genuine and based on my job - Senior Manager in Motor Claims. Theoretically, you are obliged under the T&Cs in your policy to advise your insurer that your bike has been stolen. However, this will not go against you in anyway, as you are not claiming anthing, thus NCD etc is not affected. It'll be logged as a "Nil Claim, not pursued"
If you do want to claim the value of £600 is a tricky one. If you insured on a market value basis then this figure is negotiable, if you can prove the costs of the additional work. But be warned, if the work is modification based ie zorsts etc and you haven't declared these mods then the insurer will void the policy anyway - they'll refer it automatically as you are arguing over non-disclosed mods. If you've insured on an agreed value basis, then the £600 is fixed (usually only available on Classics / really modded bikes).
My view is don't bother claiming, it's a hassle and the couple of quid you get back will be clawed back next year and then some.
One last point is the garaging clause, this may or may not apply based on whether your policy states clearly that it will be garaged - you'll find this on your insurance schedule, not the certificate. Make sure next year that garaging is not stated, otherwise you will get shafted if the worst happens again.