I know there’s guidance on tyre pressure in the driver’s door opening, but my tyre size isn’t listed.

What are people running their 275/40r20 tyres on please? Tym
(current) L322 3.6 TDV8 Vogue 2008 Tonga Green
(previous) L322 4.4 LPG Vogue SE 2003

If it sticks, force it. If it breaks, it needs replacing anyway.

Start with the pressures with the stock size and adjust to taste - if it feels light and skittish the pressures are too high and if it feels heavy and lethargic they are too low. For directional stability it's advised to always have the rears higher pressure than the fronts. It's a small size for an L322 - what's behind that choice? 2012 TDV8 Vogue (L322)

They were on the rims when I bought them. Are 50s better? Tym
(current) L322 3.6 TDV8 Vogue 2008 Tonga Green
(previous) L322 4.4 LPG Vogue SE 2003

If it sticks, force it. If it breaks, it needs replacing anyway.

255/50 R20 are stock but many people fit 255/55 R20 for a bit of extra sidewall comfort. These are standard on the L405. Ebay usually has sets of wheels with tyres from L405 owners who upgrade to 21 or 22". 2012 TDV8 Vogue (L322)