I am here and am alive!!!
Sorry for the slow responses, work and other things been making me rather busy.
Yes, you need the proper '65 AMG speedo, pic below.
Regarding your questions:
1. Some S65/CL65/SL65s came with the AMG/BiTurbo labelled dash, whereas mine is just a standard CL/S dash which only goes to 160mph. Why is this..
Maybe an early model, the very very first ones had 'normal' speedo's as they weren't ready for the UK market
2. Following on from point 1. I would like to have the car's speed derestricted (by AMG) do they still offer this service ?
Yes, but goto DMS, cheaper and better, but (possible) loss of warentee.
3. Following on again, I gather derestricting takes the car's new top speed to a restricted 186mph.. but my dash only goes to 160mph.. what do I do ?
It will still go faster, speedo will just stop. You can still use the digital speedo.
4. From cold, the exhaust has a lovely throaty roar.. but once warmed up, it's very quiet. Is this due to valve/plenums or whatever opening closing.. if so, can they be kept open ?
Nope, its the way the turbo's work which muffle the sound when engine is warm. From cold it is a lovely sound, however once warmed there is not much you can do. Aftermarket exhausts will help, but it will never be as loud as the '55 range inside. Plus the CL65 has much more padding than the 55 series.
Get someone to drive your car and boot it whilst you stand outside, its still very loud from outside!
Hope this helps!
Matt.