Had an email yesterday. It's for everyone at certain restaurants, no voucher needed. Look under general terms and conditions on link. Any day or time until 31 march. Rare for Jamie's Italian to do cheap offers like this. If you're lucky and have a gold card (free to get) and show it, ring up and book with it beforehand especially, you may also get free luxury olives to start and rhubarb and custard ice cream for dessert, as they are free at the moment every time card holders go in.
"Offer available on selected pizza specials only between 1–31 March 2017. Subject to availability. Valid in selected Jamie's Italian UK restaurants only excluding Cambridge, Covent Garden, Edinburgh, Gatwick, Liverpool, Manchester, Piccadilly, Stratford, Threadneedle Street & Victoria. Offer not available in Jamie's Pizzerias."