OK, this is the EVEN CHEAPER FIX, cost effectively nil if it is just the thigh pad sensor at fault! +++

Move the seat back and tilt the front up to gain access. Move the thigh support forwards as far as it will go.
Look underneath and you'll see a thin black cable sleeve that runs from the thigh support back under the seat, remove the cable tie to ease access.

Carefully cut open the sleeve open to confirm it contains the blue and white pair of wires for the occupancy sensor; trim back the sleeve to bare a good length of wire and cut through it nearer where it enters the pad so there's a decent length to work on emerging from under the rear of the seat.

Tape up or cable tie the spare wires going into the thigh pad, this part of the sensor is now inactive. Move the pad back now to make access easier.
Join the blue and white wires together at the end of the cable coming from under the seat.
I actually used a "choc-block" connector and small loop of wire, but that was for convenience of testing resistances. You could just strip off some insulation and crimp them together.
(I'll probably do that some time if the fix continues to work OK)
Tape or cable-tie out of the way and return seat to normal position.

Finally plug in your laptop running VCDS (or equivalent) and clear the codes from the Airbag Module.

All seems to be working normally so far...... :D

I did do some vague resistance testing, as follows:
The thigh pad was open circuit when unloaded, pressed as hard as I could manage (in some places) I could get it down to maybe 20K ohms.
The main seat pad was in the "several K ohms" range when unloaded and dropped to maybe 200 ohms (IIRC, not sure exactly what range my meter was on) with my big fat backside on it. :o

I'll report back if there are any untoward developments.....:o

And after several trips with and without front seat passengers large and small it still is. +++
No seatbelt or airbag warnings of any sort.

Before I go and remove my "choc-bloc" temporary lash-up and properly crimp the short-circuit into place has anyone with more knowledge any thoughts?

No comments on the airbag warning, but you have mentioned seatbelt warning a couple of times now... Never seen a seatlbelt fault, not even sure they were wired up on Preface ? On Facelift there are ignitors on the seatbelts to pre tension, but no sensors that I'm aware of.... Later Audis do, and by default do that annoying binging noise when you drive with no belt on...

Only in response to others comments, I don't think there are any either.

My Ford Focus of very similar vintage definitely has pretensioners though.

Hi, my airbag light has just come on. When I moved the passenger seat I think.

I looked under the passenger seat and there is a variable resistor fitted, presumably by the previous owner who experienced the same fault.

I need help! How can I get the light to go off? Do you need VAG COM to reset it.

I have the sport seats (D2 S8 PF).

See my thread elsewhere, I'll find it if I can.

If it's the front thigh pad (most likely) then you can literally just short it out as it's in series with the main one.
It will need resetting via VCDS though.

Here: http://forum.a8parts.co.uk/showthread.php?t=4873

And a few posts back here: http://forum.a8parts.co.uk/showthread.php?t=114&page=4

Once fixed you will need vag-com to clear the fault code before the light will go out.

Sometimes these things can be intermittent - mine was initially. I used VCDS (VAG-COM) to clear it and see if it came back and when/how.
After a few goes at this at closer intervals I gave up and fixed the wiring as noted above.

As yours has been "fixed" once already you'll need to look carefully to see what has been done and what might have gone wrong (broken joint maybe?).
As noted elsewhere if it's just the thigh pad sensor at fault you can just short that out.

I took the car for the MOT and the light was on, this was recorded as an advisory so the car passed. (I asked them to clear the fault but it came back on).

I can short the wiring in my own time now!

I believe that under the "new" MOT rules that were supposed to be in force earlier this year that would be a definite fail.
However, my mate who is a qualified MOT tester did mention to me just last week that implementation and enforcement is in a rather "unclear state" at present, to put it mildly. :tuttut: