Hi Glyn, sounds like you are a busy bunny lol.
If you can hold on until tomorrow, I will check the part numbers for you

There are a few differences between some of the A8 compressors (i.e diesels compressors are differents to the petrol ones, with the exception of the W12) but there have been part numbers superseded a few times over, so you may be lucky.
If the numbers are right, I would have thought that the used compressor is worth a shot ey? I'll get a price for a new one for you as well while I'm at it-just in case you decide to get a new one.
Not sur why your relay number was different though, mine wasn't?
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07 Silver D3 A/S8L Quattro

S8 front end, Xenons, Bose, Bluetooth, Voice Control, Autodim mirrors, Heated folding mirrors, Double Glazed Windows, Electric Rear & Side Blinds, Black leather Comfort Seats with front memory seats & mirrors, Front & rear heated seats, Interior/Exterior light pack, TV.I.M, AMI, Parking sensors Front & Rear, Air suspension (soon to be lowered), Keyless entry & Start/Stop, 4 zone climate control, Power Boot Open/Close.