Thanks very much for all the helpful replies chaps. I'll report back if I take it any further. One issue is that I don't have any FAA logbooks or documentation from the 'N' reg period so I've ordered copies of what the FAA has on file (for the princely sum of $4.50).
As someone who flies around 50 Hrs per year in the US and comes back to this EASA shambles each time the idea of converting is gaining attractiveness!
Obviously the overhanging risk is the one identified in IO's paper about the so-called 'Basic Regulation' whereby the bureacrats have found yet another way to choke the life out of us. I wonder how active AOPA (UK) will be in standing up for UK FAA licensed pilot's rights and the their investment in 'N' reg aircraft. (Yes I am a member).