I've retro fitted the arm rest.
I got my bits from VAG Parts and supplied some codes from the data sticker to ensure the right colour parts were ordered (the top is fabric and needs to match your seat cloth..)
I also put together a fitting guide for VAG parts, which they should supply with your order, or I can e-mail you one if you like.
I can well imagine it costing ?800 from a dealer because of the labour charges and they're correct in saying the dash has to be taken apart to fit the vents to the rear of the armrest. However, it's not worth doing all that for a retrofit, so my advice is to leave the rear vents dis-connected. It looks the same as a factory fit though.
Should take no more than 2 hours to assemble/fit and you only need a screwdriver and socket set!
Worthwhile thing to do IMO.