All,
after investigating the heavy steering problems on our car, reading hundreds of forum post here & elsewhere & finding minimal useful diagnostic info in the RR Workshop manual, I finally found some good LR info in the WM for the Jag X Type - which uses the same basic set-up.
As the potential causes & owners experience of actual causes shows nearly everything possible has caused a problem to someone at least once, I thought it maybe useful to list all the possibles here in a single post & attach a copy of the LR Diagnostic chart.
Hopefully it might help someone get to a solution to their problem faster & cheaper than many so far.
If there's causes missing, please comment.
Hope it's useful !

1. Tyres - Pressure(s) too low
2. PAS fluid - Very low level
3. Steering column Ujs - Binding
4. PAS fluid - Wrong spec (viscosity incorrect)
5. PAS fluid - Aerated
6. Auxiliary drive belt - Slipping / tensioner not working correctly
7. PAS reservoir filter - Very restricted with dirt / debris
8. Fuse for Servotronic valve - Failed
9. PAS pump / rack / hoses / pipes / reservoir / cooler - Leaking
10. PAS rack - Damaged / faulty
11. "Wiring / connector issues - BCU > Servotronic valve
.... 1. ABS ECU > BCU (Pre Facelift) or Instrument panel > BCU (Post Facelift)
.... 2. Passenger Fusebox > BCU"
12. PAS hoses / pipes / cooler - Degraded / damaged restricting flow
13. Servotronic valve (in rack) - Sticking / failed
14. BCU (BCM or GEM or Jag SCM) - Faulty (often with other symptons eg door locking etc etc)
15. PAS pump - Output degraded (pressure & / or flow)

Possible Diagnostic Checks are :
1. Tyre pressures
2. "Fault codes :
... a. BCU > Servotronic circuit failure (Resistance too high or too low) ??
... b. ABS ECU problems (Eg no / degraded speed signal)"
3. Speedometer (approx) accuracy check
4. Servotronic valve circuit current & "Duty factor" measurement via LR Testbook
(NOT possible via Gap IID BT)
5. BCU output voltage to Servotronic valve at different vehicle speeds (Servotronic must be connected) - Using Digital Multi Meter or Oscilloscope
6. Steering effort at different conditions - engine (& PAS fluid) cold, engine warm, at engine idle speed, engine revving, car stationary, car at various speeds
7. Steering effort variation as wheel turned through 360 degrees
8. PAS pump output pressure (Needs extra pressure gauge connections)
9. PAS pump output flow measurement

LR Diagnostic Chart :

Click image to enlarge

Click image to enlarge