a lot of domino's stores frequently mail 50% off vouchers to their customers, some don't however. They don't ever seem to ask for the voucher when they deliver your pizza so just ring them up and say you have a 50% off £30 voucher and if it works you are sorted, if not no harm done.
I love the double decadence base, but it's one of those love it or hate it things; give it a go if you feel adventurous.