Okay first off how many miles on the clock?
i suspect its a tank off job to check for a blocked pick up because you say it gets better when you put more fuel in it.
where are you taking it for repair Citroen or back street? What fault codes were there to replace the EGR and git it a remap?
I cant understand what you are talking about re the oil change? Jar from out the back!? what the hell are you talking about????
As Polar says and has the vehicle been serviced properly at correct intervals using correct oil not any old crap you have in the garage and fuel filter replaced. It does sound like a fuel starvation problem. Why would a garage remap when its not running properly to start with.
I'm not having a go but if you don't know much about vehicles take it to a decent garage who work on modern engines.
Spot on Tomcat and thank you re fuel filter i missed that one god only knows why my mistake, it should be the guys first port of call! But it is strange that adding fuel helps the problem and this modern diesel does have a tendency to grow crap in the tanks and fuel filters.
It sounds to me like you say major service time from someone who knows what they are doing and a decent live data diagnostic.
I'd take it to a Citroen garage & get it serviced. At least they will have the gear to fix it. otherwise you are just throwing good money after bad.