Thanks everyone for the info so far, going by the link bringing the transfer port down to 2.5mm looks a good mod but how do I do that, is it a case of taping out the 4.00mm port and screwing in the 2.5mm port? If so where can you buy different transfer ports?

I also agree a spring guide should be essential, but without machining the back block how can the spring be guided?

Thanks again ;-)