Unfortunately Paul you'll need to send them actual evidence of you at work ie invoices, paperwork, witness statement from colleague or manager?, Cctv. As well as a statement from the garage to testify the car was there. Plus I'd like to think they'd have CCTV.
I'd give them the police crime reference number.
Unfortunately you are now guilty until proven innocent in the UK. It's safe the works has come to this, but our authorities trust no one. Yet they expect us to trust them!
Cheers
Steve