[Yobbo]

Well the latest update as above is that I'm now running S7 motorsport shafts (though I actually believe they are the latest S9's) and raised the car to -25mm.
Wobble is still present but on the first few 2nd gear pulls of the day in this cold weather it actually doesn't actually appear, so only when the car is warm...

Over the last week I came across some posts on a Focus ST forum and also on Swedespeed which detailed worn upper gearbox/engine mounts causing vibration as suggested - when accellerating from 1-3k rpm in low gears (peak wobble most pronounced in 2nd gear at around 2k for me).

This is the mount located under the battery box so the strut brace and all that has to come off to get access.
You will need to eventually have the car on stands and the engine supported with a jack using a plank of wood on the sump - easiest to remove the battery first while car is on the floor!!

In any case I did find some wear on mine an found a notable difference in the design of D5's transmission mount (otherwise known as 'left engine pad') compared to the petrol variant of the 5-cylinder engine.

What stood out to me was that instead of a shallow rectangle design as found on the Focus ST/T5s the mount had a much larger rubber cone -
Volvo part no: 30741143
I'd been aware for some time that the D5s do not have a gearbox lower torque mount and associated arm - for what reason I do not know, perhaps it's because of the way the PAS pipes are routed hence the bracket which connects them to the side of the M66 gearbox. However, to me it seems like a seriously ill thought out design consideration when you factor in how much torque a stock D5 can make!
This seems to be a possible factor as to why the D5 mount is designed as such;
a) to reduce NVH associated with a diesel engine
b) to make up for the lack of a lower gearbox mount!!

I spoke to a guy called Dom who'd had a C30 D5 with the exact same symptoms, he purchased a new set of shafts but the offside driver's failed not at the inner CV joint but at the support bearing just like his old one.
The new shaft lasted just 30k and in the end he moved on and got an Audi S3.
It makes sense to me that if his top gearbox mount had deteriorated, you're going to get so much movement from the engine and box, and seeing as the support bearing is fixed in place it's going to take a hell of a hammering.
In my case it may be that the box and engine are moving so much that its allowing the inner CV tripod to reach such an angle that its on outer range of the cup.

Now the reason why the J&R uprated offside driveshaft is not producing the vibration is possibly down to the fact it does not use a tripod bearing but rather a 6-ball bearing Rhzeppa joint which allows a much greater degree of angle for operation. I did a little test with both types of shaft off the car and the J&R one could indeed allow a much greater angle without being notchy at all, wheras the tripod bearing variants were when at their maximum angle value did notch as you rotated them. (imagine holding the shaft up at 45 degrees and rotating it - that was my test haha!!)

So I have Febi Bilstein top gearbox replacement ordered from Autodoc on it's way and fingers crossed, this will be the cure!
If it is then I'm also going to look into retrofitting a lower gearbox mount to assist with things - it's only going to be of benefit.

Will report back...