I finally managed to get around to doing this. It was getting worse over the last few months and the wood cover over the relays is looking and feeling quite wet so may need to replace it if it doesn't dry out well enough.
Thanks to all the help from people on here I was able to locate the offending hole with only a little difficulty! I used an M8 rubber well nut and it fitted just right, slightly larger might fit too.
I then used a long allen key and could reach through to tighten it without needing to remove anything.
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Cars...
'93 Audi Coupe 2.0 Alpine White (sold)
'99 Audi S4 Merlin Purple - K04 upgrade to 412hp (sold)
'00 Audi S8 Agate Grey (Sold
)
'11 Audi Q7 4.2tdi Unknown Red (Classic Red Pearl Effect - thanks MJ)