The option to buy your bike back is down to the insurance company. Basically they don't want the agro of getting rid of it so sell it back to you. Its up to them if they offer this. Not all do.
In your case, they have admitted liability. Just argie the case to get a good valuation for it based on dealer adverts for similar condition/mileage bikes. Send these to the insurance company and push for they average of the advert prices less any excess you may be liable for if the accident was your fault. In this case you say it wasn't your fault, so push for the average replacement cost from a dealer based on the adverts you can produce from reputable sources. (bike trader/e-bay classifieds etc).