Well I've read with interest a few of the non starting issues on here.
However I'm not sure mine is exactly the same problem.
A couple of months ago I got the "HDC disconnected" message when the car failed to start. Always started 2nd try. This then became more frequent, every other start up, but never failed to start at the 2nd attempt.
Then last week while reversing, the sensors stopped beeping, the sat nav display cut out, came on again, cut out, then the beeping went haywire. Put it in D and still the same. Turned the car off, started it again and all was fine and dandy again. However the parking sensors seem to have a mind of their own, sometimes working fine, or one continuous beep even when I was nowhere near anything at all when parking. The intermittent starting has now turned in to not starting at all. Went out last night to drive home, usual half start then nothing. Tried the usual 2nd start but the brake pedal (auto) is solid. The checks now come on , but when the brake peddle is pressed it's now constantly solid, with no downwards movement. Had to walk home 3 miles last night and walk back again this morning. (Quite a pleasant walk , even in this weather). I took a battery pack with me but to no avail. The pedal is rock solid and now the steering seems to be locked too.
I get no error messages. Just the initial first checks then nothing when I try and depress the pedal .Everything goes blank.
I need to shift the car as it's parked across the other half's drive and she needs to get her car out in the morning for work. I can't just push it out of the way as it will block someone else in.
Any ideas ?

Ok so I'm £100 down. Just bought a new battery thinking this may be a contributing factor. No. In fact I'm now getting the "HDC fault, system not available" constantly now along with a warning triangle at the bottom of the screen that wasn't there before. So I have a dead car that needs to be shifted by the morning. Great !
It's now pitch black outside .Typical, can't even see what I'm doing. Tried to take a picture with my camera but it won't capture the wording, just shows a green rectangle.

Ok, so the steering isn't locked. I do however get a single beep before everything shuts down.
Get the feeling this might be the starter motor itself . Might try the "earth lead" mod in case it's bad earthing. However on the other threads I've read, this relates to poor starting and HDC fault message rather than complete non starting and HDC message on constantly.
I believe the starter motor on the i6 lives at the bottom rear of the engine.Does anyone know how easy/hard it is to access? Anyone have pics ?

I don't have any concrete suggestions, but the solid brake pedal would concern me - why is it solid? Since the pedal must be depressed to enable starting is this also the cause of the non-start? Is the HDC error message related to this? Sounds like you could have a problem with the braking system which is having knock-on effects. Given the potential safety implications I'd want to check that out.

The parking sensor problem could be unrelated - they are prone to the kind of behaviour you describe if they get water in them, and it has been a bit wet recently.

Brake pedal, I now believe , may have been stiffer due to constantly trying to start the vehicle. There is enough movement for the gears to be selected. I think it may have been pumped a few too many times. Like any brake pedal,if you give it enough pumps it will stiffen up.

It still sounds like a power related issue to me. The "HDC not available" message is the go-to-error for when battery power is low during cranking...

Do you have a code reader or some other device which can get the voltage at the OBD port? I'm thinking that although you've put in a new battery, the required voltage might not be making it to the vehicle systems, hence the errors.

Bad earths is a possibility, of course. Could be something else too.

Did you actually measure the voltage of your old battery? What about the new one? (I bought a "new" battery a little while ago, turns out it was 4 years old and had been sitting on the shelf the whole time, it was also stuffed!) 2008 Freelander 2 HSE Si6 Black/Black - Hers
2009 Jaguar XKR 4.2 S/C Portfolio
2005 Jaguar XJR 4.2 S/C Black/Cranberry
2005 Jaguar X-Type 3.0L AWD - Gone

The battery is brand new , fully charged and comes with a 3 year warranty. The old battery was a Halfords job that wasn't the correct rating, but something Halfords list as being correct for the i6.
I believe that the starter motor is the culprit. Why ? Well I took the sump shield off , cleaned the terminals and gave it a few hefty whack with a hammer and the Freelander burst into life. However all is not good. HDC messages were still showing . I drove for 100 yards and the thing would not rev over 3000RPM. It sounded as rough as arseholes too. I turned it off. Started it again, all the HDC stuff was gone and the light restored at the HDC,TC dial. Great ! Or at least I thought it was. It still rough idles and won't go above 3000 rpm. It is very slow to change gear. Feels like the brakes are being applied.