Martin, removing the DPF is an automatic MOT failure. I think it is an EU directive that was brought in this year about February.
There is some argument about how successful the DPF is as I know of someone who had it removed and the emissions were better than previously.
I have been given very reliable information that the DPF inspection is a visual one, and will be spotted if the DPF is replaced by a straight through pipe. However, if the DPF is removed, the innards taken out, and then rewelded it can pass cursory inspection.
However, the cost of putting it back in is around £2k, I believe, so my solution is a good burn up once a month or so. About 20 minutes at over 50, making sure that the engine has got up to full operating temperature first .
Peter.