So I filled up today and found I got 36.1mpg from thus tank, 3.5mpg more than last time. This could be due to the additive, so e longer runs, cleaning the egr or more likely a combination of the 3, either way I'm happy it improved. The additive shouldn't really show improvements to mpg until after a few tanks so fingers crossed it will continue to improve. It has been very cold here too which hasn't helped the mpg.
Also I think I need to reset the ecu properly. The readout on the dash shows 8.7 litres/100km which Is nowhere near 36mpg!