Thanks, one thing I noticed as I was going through the application was that there is no actual confirmation that the discount had been applied. Once I'd got the final confirmation email from them I emailed them through the customer support page to query it, quoting my booking reference. They replied 2 days later to say the discount had been processed and applied. So, not sure if you need to chase it or not or if they genuinely automatically apply the discount. I may be a bit cynical but reading their reply I think it may be wise to chase it up.
Dear Mr E****
Your Eurotunnel booking reference number has now been successfully validated on your account
The discount will now be applied to your Liber-t account.
Yours sincerely,
Customer Services