I'm sure me bringing this thread back to the top of the pile will inevitably result in a few groans from those subscribed to the topic, however for what it's worth it's actually to provide some closure on the subject!
I've been in discussion with Viessmann technical support who I must say have been extremely helpful and seemingly more than happy to get stuck into discussions of the options available, and the intricacies thereof. I got in touch with them with some trepidation fearing that they may only be willing to give the time of day to RGI's and the like, but they couldn't have been more helpful.
As many of you have been saying, they do indeed recommend the use of the W-plan configuration, however it is not exactly as per the standard Honeywell design. This is where my concerns were coming from because, as I attempted to demonstrate, the default W-plan wiring layout is inadequate given the suspected result of the nuance of weather compensated boiler operation in regard to its HW priority provision.
Viessmann confirmed that with a standard W-plan implementation there could indeed be a catch-22 situation in the scenario where
all of the following are true:
1. There is a demand for CH (by both CH timer and room stat*) (*or results from the outside sensor if used without a room stat)
2. There is no demand for HW by the HW timer
3. The temperature of the cylinder is below the target set by the cylinder stat
4. The weather compensation algorithm determines that a less-than-maximum flow temperature is required
In this state the system would effectively be 'trapped' in a catch-22 situation whereby the boiler is running in weather compensated mode and the diverter valve would never switch over to the CH circuit because the boiler's flow might be of a lower temperature than required to satisfy the cylinder stat.
As per my suggested modifications to the standard W-plan (which at least one person admitted to not even having looked at - it's no wonder our discussion didn't really get anywhere!

) it is imperative that a HW timer
with changeover contacts is used such that there is an explicit 'HW is not required' output available. This allows the CH demand to be satisfied without attempting to satisfy the cylinder stat first (which it might never do as discussed).
As an alternative to modifying the standard W-plan wiring, they did say that you could use a combined CH/HW timer of the variety that does not allow CH demand to be called without HW (i.e. three states: CH off HW off, CH off HW on, CH on HW on) but he said sticking with my CM927 would make far more sense and so I should just swap my timer (Sangamo PSW) for one with NC/NO changeover contacts (e.g. a Honeywell ST9100C).
To ease common understanding we based our discussions and correspondence on the wiring diagrams provided with their outdoor sensor kit (as Mysteryman recommended) and so I thought it might be useful for anyone else not fully conversant with this particular kit and variation from the conventional W-plan wiring that I make them available here:
Weather Compensation Outdoor Sensor Kit Instructions
Cylinder Demand Terminal Box Instructions
After all this I hesitate to admit that I am actually now starting to have second thoughts about having the weather compensation kit fitted at all because I am very doubtful of its ability to prove cost effective. At a cost of ~£180 for the sensor and cylinder interface box and promises of only 2% greater efficiency it is going to take 20 years to recoup the outlay given that my annual gas bill currently resides at only £450! To his credit, the chap from Viessmann did say that the benefits are more to do with comfort rather than cost-savings (despite the advertising blurb bias being the other way) and that cost savings would be greater realised for much larger properties/consumers. He also said that had I not already been using a TPI controller that can at least approximate a modulated demand and the comfort and control benefits that this brings that it would still have been of noticable benefit.
Thanks everyone for the discussion - I may have gone full circle but it was a beneficial journey for me and if nothing else it has cemented my intent to go with the 100W Compact.
Cheers all,
Mathew