Firstly, you can't work out anything by putting £30 in because you don't know where you're starting from. You need to do at least one brimful to brimful calculation.
Secondly, how long are your journeys? 30 zones are typically full of traffic lights, roundabouts and junctions which all kill MPG. B roads are ok if they're free-flowing, but if you're accelerating and braking all the time, that's going to kill it too.
I wouldn't even bother looking at your EGR temps until you know what your MPG really is.
Just how many Yorkshire sheep can you fit inside one exhaust?