Have just fired off an offer. Thanks again guys, no part ext to consider and I'd be taking them up on finance so they'd be making money off me from that too. Fingers crossed, haven't driven a GTS yet, but from my brief experience of the base model GTS will suit me just fine.
Had unofficial chat with another dealer and they sort of confirmed Cayman 6 is coming but will be very limited numbers hence availability. Kinda makes sense when you think about it for any length of time. Won't be GT4 performance, so maybe for me the F4 is the way forwards.
< edit > dealer called within 5 minutes of email having been sent to advise offer refused, and that wiggle room is very small 'priced to sell', so we'll see what happens. I have a feeling this could be a couple of weeks of negotiating ahead of me - assuming no-one else gets in there first.