Beware that video is out of date, the law changed in October 2012. Now they don't have to find the driver; they can claim from the owner even if they don't know who the driver was. They do have to follow strict new procedures though.
Since October the best defence is to either prove you didn't see a sign or establish that you couldn't reasonably be expected to see the sign, or that it is not resonable to think that their sign relates to where the car was parked. Therefore you have not entered into a contract with them. Vine against London Borough of Waltam Forest indicates that to establish a contract you would need to demonstrate that the car driver actually saw and understood the significance of a warning notice or notices (
http://www.bailii.org/ew/cases/EWCA/Civ/2000/106.html). Judges however are likely to decide you did actually see the sign (despite what you claim) if it was reasonable for you to have seen it and understood it.
If that fails tell them their ‘fine’ is an unfair term in a contract, contrary to section 5 of the Unfair Terms in Consumer Contracts Regulations 1999 (below)
“5.—(1) A contractual term which has not been individually negotiated shall be regarded as unfair if, contrary to the requirement of good faith, it causes a significant imbalance in the parties' rights and obligations arising under the contract, to the detriment of the consumer.
(2) A term shall always be regarded as not having been individually negotiated where it has been drafted in advance and the consumer has therefore not been able to influence the substance of the term.
(4) It shall be for any seller or supplier who claims that a term was individually negotiated to show that it was.” Untested as yet, if you were not the driver I don't see how you could see the contract to accept it.
Failing that tell them the fine exceeds any conceivable loss or damage caused by the alleged trespass, especially if no marked bay was occupied by the vehicle and the road was not obstructed. It is very doubtful that merely the costs incurred in policing the area and enforcing the fine can justify the fine itself.
If they hassle you or send debt collectors round this can be classed as harassment. Regard any contact with debt collectors, or by their debt collecting department, or warnings of the involvement of debt collectors, as unreasonable and unlawful harassment as per section 40 of the Admin of Justice Act 1970:
“40. (1)A person commits an offence if, with the object of coercing another person to pay money claimed from the other as a debt due under a contract, he—
(a)harasses the other with demands for payment which, in respect of their frequency or the manner or occasion of making any such demand, or of any threat or publicity by which any demand is accompanied, are calculated to subject him or members of his family or household to alarm, distress or humiliation;
(b)falsely represents, in relation to the money claimed, that criminal proceedings lie for failure to pay it;
(c)falsely represents himself to be authorised in some official capacity to claim or enforce payment; or
(d)utters a document falsely represented by him to have some official character or purporting to have some official character which he knows it has not.
(2)A person may be guilty of an offence by virtue of subsection (1)(a) above if he concerts with others in the taking of such action as is described in that paragraph, notwithstanding that his own course of conduct does not by itself amount to harassment. “
The design and wording of the initial ‘parking ticket’ is probably already a breach 40 (d) above, in that it will I am sure resemble an official public parking ticket much more closely than is necessary or reasonable for its purpose.