As far as I'm aware it doesn't matter about the livery or indeed the siting of the mobile trap. If they get numerous complaints about sitings they maybe lose their entitlement to keep the money generated but I'm afraid it doesn't make your ticket invalid.
There are a few things you can check such as did the NIP arrive within 14 days? Are you sure it was you driving? If you genuinely don't know who was driving you can use this as a defence but you must be able to back up why you can't remember. This might be becuse you have several named drivers or that you leant your car to different people.
The guys over at
www.pepipoo.com may know more, and it's definately worth checking out their FAQs.