I saw signage up for this yesterday in Cardiff, and it was strangely gone this morning so I asked about it. It starts on Monday, the staff don't know how long for exactly.
So buy a meal deal as normal, keep your receipt and then you can pick up another in the same week for £1.99. They couldn't tell me if you can do it on the same day i.e. take two meal deals to the till and pay separately to get the discount... I guess we'll have to find out when it goes live.
Hopefully we can combine it with the BOGOFs on drinks too!