Presumably as they're sending you a claim form they're admitting some form of liability. If you know it was a council van, then hopefully you can describe it, make, model, signwriting details etc - and they'll have a record of the van's movements so there's no reason to believe you're 'stuffed'.
You don't really give much away. Car park or private area? Did you witness it? Did anyone witness it? Has it done much damage etc