Who it applies to
Tenants who were already in the property. A tenancy entered into after the changeover does not need the Information Sheet; it needs the written statement of terms instead, which is a different document with its own deadline.
If you missed the deadline
Serve it. A late document with a dated record is a far better position than an absent one, and this is a breach rather than an offence, which means a civil penalty rather than a prosecution.
Proving it
Send by email with the current PDF attached, or by tracked post. Either way the send is logged with the time, the recipient and the document, and the version attached is the current published one rather than a copy somebody saved last year.
The number
Up to £7,000, for a document you could have emailed in an afternoon. That is the whole risk calculation.