Hi Simon,
Do you mean that even if I major overhaul a page (not in a CMS) where it uses the same url - I should still do a 301? Ive never heard of that - or do I misunderstand you.

Im not afraid of CMS's - I use 3 now (WPress, CMSMS and another) - I am really comfortable in customising them and writing my own UDTs and chopping them etc, thats not my issue.
This site as I said is top of our niche - a big ask in any case. Even with 301's - i want to be gentle about changes so as not to drop us off the planet - double guessing what google will think of a template to cms makeover is anybodies guess - including mine.
My guess was that if i go CMS and do 301's - we will still drop out cos of the reaction by the BigG bots - am I wrong? Maybe we come back - but after how long. Ok - Im paranoid - but ....
Regards