An update...
I had a chat with Mondial/Mercedes warranty today regarding this situation that won't seem to go away for me.
The good news is it looks like things are on track now. Hopefully this thread may help a few buyers in future, though of course warranty companies change their terms and conditions from time to time.
I was told that the independent service is not an issue as it was outside the warranty policy period, but that recommended fluids must have been used for that service.
Fortunately the specialist in question (M-Only, West London) used the correct Mobil 1 OW40 oil during the service and I have a copy of this invoice, which I will be forwarding on to Mondial for them to put on the file. Using the wrong oil would jeopardise a claim relating to the engine, apparently.
C240Sport97 has been very helpful in this process, assisting me with the legal side to ensure I cover myself in case of questions later on if I had to claim on the policy. Mondial record telephone calls, but C240Sport97 is pointed out that I should write to them with my understanding of the situation (in order that there is a record of it for them to respond to, should they disagree).
I've not even seen the car yet believe it or not(!), but now with this peace of mind I will go and take a look with a view to buying, and I'm confident it's the one based on the hi-res pics I've seen.
I believe for many of us a proper mercedes tier one warranty on the SL55 is gold dust, as my own research tells me that these are complicated cars, to say the least!
Hope to update soon once I have my hands on the car....