If you have Dawn dishwashing liquid in the UK it is a good soap that breaks down oil fairly well. I'd remove the trunk carpeting and mix up a bucket of water and dishsoap and clean with a soft brush. remove the water / soap with a wet dry vacuum. Repeat as necessary.
Might be easier faster just to replace the trunk carpeting. Guys who now have dedicated track cars might have a set of trunk carpeting laying around.
98 C2 cab tip glacier white / black top
"I Don't Care - my Tip is a Blast!" -thanks Dave
Official Blue Flagger HWFM Racing
On the trunk floor, there are a couple rubber plugs that can be removed to let water drain out as you rinse. The best thing to use is a fine mist. After the rinse, wipe and vacuum dry the excess water and immediately use a blow drier.
1996 911 turbo (993tt)
1987 911 turbo (930)
1985 911 Carrera
1972 911 T
I would use Wurth degreaser and bolt up the oil. Apply a small amount of solvent a white cloth. Dab repeatedly, then press the cloth on the spot for a few seconds. Rinse by blotting with a white cloth soaked in lukewarm water. Wait one hour or so, until the carpet completely dries. If the stain is getting lighter, repeat until the stain is removed.
If oil is not dry and still wet, maybe try something like kitty litter to dry the oil, and vacuum up as much as oil as possible. I would also stay away from soaking with water if possible as it may spread.