Jason,
No you cannot say you don't know who was driving the car at the time, you HAVE to provide name(s). The way to do it is to first ask for photographic evidence (which you are entitled to). If upon receiving this you are unable to clearly identyfy the driver, then you STILL have to provide a potential name, so look back at your time sheets to see who was working on that day and then provide them with a copy of this info, saying that it could have been any of these people, but as you do not keep a log of drivers (which you are not legally obliged to), you're not sure who it was, as all of them (are naturally) denying it was them. Then point out that this has highlighted a flaw in your system and advice them that you will be establishing a new company policy that will ensure that a log will be filled so this doesn't happen again.

As it is such a small speed, it would be pointless them pursuing it through the courts.