In order to prove guilt the prosecution must prove a valid requirement under section 172(2). A requirement is valid if:-
(a) a driver of a vehicle is alleged to be guilty of an offence identified in section 172(1);
(b) the requirement is made of a person identified in section 172(2) i.e. that person is either the keeper of the vehicle or "any other person";
(c) the information to which the requirement relates is either such information as to the identity of the driver as he is required to give - or any information which it is in the power of the addressee of the requirement to give and which may lead to the identification of the driver;
(d) the requirement is made by or on behalf of a chief officer of police.
"Although the statutory provisions are silent as to the form of any section 172 request, any request has to be reasonable, with reasonably adequate details as to whom, where and by what means the information must be provided.
There is a presumption that “the keeper” of the vehicle for the purposes of section 172(2)(a) was the keeper registered as such with the DVLA at the relevant time.
When a person gives his details to the DVLA as the registered keeper of a vehicle he assumes responsibility for responding to correspondence and notices relating to that vehicle sent to him at the registered address.
Section 7 of the Interpretation Act 1978 provides that unless the contrary intent appears - service is deemed to be effective by properly addressing, pre-paying and posting a letter containing the relevant documents and, unless the contrary is proved, to have been effected at the time which the letter would be delivered in the ordinary course of the post; by section 172(9) that is satisfied by sending a notice to the person’s “last known address” in respect of a section 172 requirement notice. However, there is a presumption that the last known address of the registered keeper will be the registered address of that keeper and there is a presumption that service on a registered keeper is effected by posting correspondence or a notice to him at the registered address. Of course, those presumptions are rebuttable.
Where a section 172 notice is sent to a registered keeper at a registered address that keeper will be guilty of an offence under section 172(3) if he fails to provide the requisite information within 28 days unless, on the balance of probabilities, he can establish a statutory defence under section 172(4) or (7)." R. v Lynes  EWHC 1300.
Section 172(4) - Reasonable diligence
A person shall not be guilty of an offence by virtue of section 172(2)(a) if he shows that he did not know and could not with reasonable diligence have ascertained who the driver of the vehicle was. This defence applies to the "person keeping the vehicle" rather than "any other person"; see R v Grant  EWHC 1114. Reasonable diligence to ascertain identity falls to be assessed at the time the request from the police was received. Reasonable diligence generally means questioning all potential drivers and investigating the whereabouts of the same at the relevant time.
What is "reasonable diligence?"
"Diligence" is defined by the Oxford English Dictionary as:
"Constant and earnest effort to accomplish what is undertaken; persistent application and endeavour; industry, assiduity"
"It relates to accomplishing an undertaking, that with which one is tasked. It cannot exist in a vacuum – but must relate to something specific. If there is no task to accomplish which is in mind, it is difficult to see how conduct can be described as "diligent". To this extent, the word is unlike one such as "prudent" or "reasonable"; see Atkinson v DPP (2011)
Section 172(7) - Not reasonably practicable
The person on whom the notice is served shall not be guilty of an offence under this section if he shows either that he gave the information as soon as reasonably practicable after the end of that period or that it has not been reasonably practicable for him to give it. This defence often arises where the defendant contends that he did not receive the request.
Section 172(2)(b) - Not in his power to give
If the defendant wishes to contend that the information required was not in his power to give then he should raise that issue by asserting that he was not the keeper. Alternatively, that the vehicle was elsewhere at the relevant time. If the defendant is not the keeper and does not reply at all then he is guilty; see Grant. If he does reply then the prosecution would have to disprove that he was not the keeper, or prove the location of the vehicle if that is an issue in dispute. The prosecution would also have to prove that the information was in his power to give; Mohindra v DPP  EWHC 490.
Information was supplied
It would also be open for a defendant contend that he supplied the information asked of him. The prosecution would be required to prove that he did not comply with the section 172 requirements.
This offence can be dealt with by a £200 fixed penalty and 6 penalty points but it is more common for the matter to proceed to court where the fine is anything up to £1000.