I'd just return the form and if anything else arises from it then get some legal advice. Most solicitors will offer a free 30 minute consultation to asses whether you have a case worth pursuing or not so that should at least help if needed. It won't be cheap if you do take a solicitor on after that but if you're innocent then it's worth doing. I was late in returning a speeding letter many years ago as they cocked up the postcode, they did chase it up so it's best just to play ball at this point I'd say.
Best of luck
E93 320d Msport - sadly gone to the salvage yard.