Declan, you have made your bed, you are now going to have to lay in it, same as P0420 on petrol cars, you have fitted and charged for inferior parts, up to you to sort it at your own cost, not customers fault you fitted sh** parts to their car,but it will be up to you to sort it out and no court in the UK will support you, buy genuine emission parts, I get fed up up telling traders this !!!!!