Thanks for bringing this to my attention. I think the change to their new responsive website may have caused it to stop working as I just tested it myself and while it kind of registered, it did not actually apply the discount.
Will keep you posted.
Update from MRL by: Paul
I just heard back from MRL and they advised that the coupon code is not actually applied until the final page in the checkout..
They said they are working on streamlining the checkout process so it will show up earlier and also advised that if for some reason you miss the discount, you can email them and they will credit your account with them - and you can also phone in the order and mention 'sbgphone' and you will get the discount over the phone.