I looked into this about two years ago and came across another option, namely the jets used in a similar aged A6. The part numbers are 4Z7955101 (passenger side) and 4Z7955102 (driver side). The relevant A8 part numbers are 4D0955989A (passenger side) and 4D0955990A (driver side). All of them are NLA, but you can get ones from the likes of AliExpress.
Here's one on AliExpress similar to the ones I bought for both left and right jets for under £7.00 (just select 'Pair' from options):
https://www.aliexpress.com/item/32974381649.html
Audi Tradition have the jets for under 24 Euro each and the caps (4D0955269 passenger side and 4D0955270 driver side) for under 30 Euro each, but you need to find a way to get them delivered to the UK.
https://shops.audi.com/en_GB/web/tradition
EDIT: Forgot to say, but you can replace the jets in situ.
__________________
2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
Sold! 
2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds