If you have a legitimate complaint and have the supporting documentation, you can send a detailed description of your complaint to...
Customer Relations Team
They will reply within 3 working days with an acknowledgement. And their investigation will last a maximum of 28 days after which they will get back to you. If you are not satisfied with their response to your complaint, you can escalate. An explanation of how to escalate comes when they reply.
It's best to send your notification of complaint as soon as possible.
If your complaint does not require supporting evidence and you expect the investigation to be straight-forward, you can simply email them at email@example.com
If they decide to remedy your complaint with compensation, they will use the relevant UK consumer protection guidelines and they have the option to compensate in kind.
Update 2 September 2015
Eurotunnel delays are becoming increasingly severe, and this may prompt them to revise their complaints procedure or to be more captious in processing compensation claims.
If this eventuates, Travellers should be advised that claims will need to be rigorously documented and the 'small print' may contain newly worded exculpatory clauses.
See also: http://www.bbc.co.uk/news/world-34125950