It would appear the use of the discount code is restricted to certain accounts. You are prompted for the code when signing in to your online account, so I don't think it would work on any other WebTIS powered site.
The price appeared to only have the 10% discount applied when I booked it. I was hoping the new price might kick in prior to payment, but I went through Verified by Visa and realised it hadn't.
I've emailed East Coast, so will hopefully be able to find out if it was a technical glitch or something.
My ticket was an advance ticket bought through East Coast, so they would know what train I was travelling on and that it was (significantly) delayed.
Here's the contents of the email for anybody who's interested:
Service Disruption
Dear Customer,
We would like to apologise for the delay and disruption to your journey when travelling with East Coast over recent weeks. Regrettably the adverse weather and overhead power line problems have impacted upon our services.
As you were delayed you may be entitled to claim compensation under our Delay Repay scheme. Please visit our website for full details and to make a claim.
By way of an apology, we would like to offer you 20%* off your next purchase with us. Book online by 31 January and you will get 20% off our lowest Standard and First Advance fares.
How to book:
1. Sign in to Your Account
2. Enter your exclusive code: DISCOUNT
3. The discount will then be applied when you book.
Once again we would like to apologise for the disruption to your journey with us and we look forward to welcoming you back soon.
The East Coast Team
*T&Cs apply. Tickets are subject to availability. Cannot be used in conjunction with any other offer. Discount already includes 10% online discount where relevant.