I have a XF 2l AWD sport. For 2 years I have been topping up the def fluid with fluid from Halfords with the correct iso number. The car recently displayed a message that incorrect quality def detected. I took it to an independent Jaguar specialist who said it would need a new def injector and be refilled. He said it would take 2 days as he had to order the injector. After fitting that he now says the car is showing another fault he doesn’t know what it is that’s wrong. He is blaming 
me for using Halfords fluid. He has now had it 5 days and it still not fixed any ideas as what else could be wrong as he doesn’t seem to know

If the Halfords fluid has the Number recommended by Jaguar on it it should be alright and providing it's not over 2 year's old because they say that it only has a shelf life of 2 year's.

They recommend to replace the Ad Blue fluid if it's 2year's or older, problems can happen if there's Crystalisation in the injector and/or the Tank.

Some say that cleaning the injector and draining the Tank (which holds 17 litres in the XF ) can be a DIY job.

If you look at Red68's thread in the XE forum it may give you some more information.

Regards Tom