OK Finally managed to get this done today. Once finally managed to get the wiper arms off, process was this:
1) Remove foam rubber seal at front of the plenum cover, just pulls off easily
2) remove plenum cover this is secured at the back via a "tongue and groove" type fixing along the length of the cover, Start at one end from underneath and use something to push the cover up. I used a hammer shaft and then flat bat to lever it up. Was quite fiddly as theres not much room to the edge of the bonnet as it starts to seperate. Once you get the end seperated the rest is pretty easy. I Then cleaned the tongue and slot to make sure they would clip together again easily
3) Remove the 2 star bolts holding the motor assembly in place and the electrical connection. You cannot remove the motor assembly straight away as its too close to the plenum bulkead so you need to create some extra space
4) I didnt remove the whole bulkhead just worked my way from the left hand side (UK driver side). To get to the fixings you have to loosen the accoustic felt by removing to wide fixing, then there are some star bolts, a plastic nut that holds some electrics and a double ended bolt with 10mm nut in middle (see pic for what fixings I removed)
5) Pull bulkead forward and wriggle the motor assemble free, there's a location peg at the back as well.
6) Fit replacement motor assembly, bolts and electrical connection (TEST BEFORE GOING FURTHER)
7) Re-Fix bulkhead/felt

Fix plenum cover back in place and push foam front strip back in place
9) reattach wiper arms and align
Left hand rorating spindle on old motor assembly was really seized, impossible to move by hand, definately worth buying a 2nd hand spare (latest version "c") to have regreased and ready to fit
Some pics I took whilst doing it
Lip on underside of Plenum Cover

- Lip on underside of Plenum Cover
Locator groove for lip on underside of Plenum cover

- Locator groove for lip on underside of Plenum cover
2 Hex key bolts and location spike

- 2 Hex key bolts and location spike
Pull felt forward once retainer and plastic nut removed

- Pull felt forward once retainer and plastic nut removed
Location of Bulkhead fixings I removed to allow it to be moved

- Location of Bulkhead fixings I removed to allow it to be moved
Left hand side of Belkhead with fixings removed

- Left hand side of Belkhead with fixings removed
Felt & Bolts removed from LHS of Bulkhead

- Felt & Bolts removed from LHS of Bulkhead
LHS motor assembly bolt

- Left hand side of Belkhead with fixings removed
RHS motor assembly bolt

- RHS motor assembly bolt