Thanks again for all the replies, I think using the old DPF housing to asses the emission level and smoke testing the inlet is a good idea.
I also think DPF removal my be a cheaper and easier option for the customer to consider.
I hadn't considered doing a forced regen on a new filter but it is worth a try, thank you for sharing your experience, talking with the customer this afternoon. I'll keep you informed with the out come.
Kind regards, Andy