If you're after Portland Stone in a Dulux tin, your two best bets are Dulux Fresh Artichoke (LRV 51, ΔE 1.9) and Dulux Crushed Aloe (LRV 50.8, ΔE 1.9). Both land at ΔE 1.9 from the original, which is within the "very close" band — most people standing in a room wouldn't clock the difference, though a careful eye in raking daylight just might.
Why two matches at the same ΔE? They're approaching Portland Stone from slightly different angles in colour space. Crushed Aloe leans a touch greener, Fresh Artichoke a hair warmer. If you've got the original Little Greene chip, hold both Dulux swatches against it on the actual wall — north light and south light will pull them differently, and the one that disappears against your Portland Stone reference is the one to buy.
A word on what you're trading. Portland Stone is one of Little Greene's lovely soft greened-stone neutrals, and part of its appeal is the depth and pigment quality of the paint itself, not just the hex value. A ΔE 1.9 Dulux match gets you the colour very close, but Little Greene's Intelligent Matt and Absolute Matt have a particular flat, chalky richness that a Dulux finish won't replicate exactly. If it's the colour you're chasing for a big job and budget matters, the Dulux match is a sensible call. If it's the finish and feel you fell for, sample both before you commit.
Practical advice: buy tester pots of both Fresh Artichoke and Crushed Aloe, paint two coats on a bit of lining paper, and tape them up next to a Portland Stone sample. Live with it for 48 hours across changing light. Cheaper than repainting a whole room because the match looked right on a 5cm chip in the shop.