Thank you all for the replies.
Alfa.Romeo, thank you for the offer. I'll see what he says but I'm afraid I'll probably end up causing more problems or damage for him!
Silverthorn2151, thank you for that. I am aware of this and the original DPF is with the car anyway, so it's not a major concern at the moment.
JabawokJayUK, I've called up a couple of garages and they're a bit indecisive as to what it could be (and quite rightly so) without doing a proper diagnostic. One specialist that I normally use said it could be the shift gate, or any number of sensors, or worst case, the ecu itself. Another has said they've only ever had one car with a gear problem for which they had to replace the shift gate, but the symptoms were different, i.e. the lever wouldn't move from 'P' or something and the key wouldn't come out or something like that.
I guess it's safe to say we'll have to walk away from this one.
Thanks for the help.