Reach for Dulux Smokey Slate. At a ΔE of 2.1 from the original it's a very close match — close enough that on the wall most people would never clock the difference. Dulux Thunder Clouds comes in second at ΔE 2.5, still very close but a touch off.
Down Pipe is that brilliant strong leaden grey with a green-grey undercurrent — it's neither cold steel nor warm taupe, which is exactly why it's a pain to match. Smokey Slate (LRV 12.3) nails that depth and the slightly mineral, slatey character. Thunder Clouds (LRV 14.3) reads a fraction lighter and a hair cooler, so if you want something marginally brighter on the wall it's a sensible alternative, but Smokey Slate is the one I'd specify first.
The usual "but what about" here: will it actually look identical? Honestly, no two paints are perfectly identical once you factor in different binders, pigment loads and sheen. Farrow & Ball's chalky, deep-pigment finish has a way of holding shadow that Dulux's formulations render slightly differently — that's the part a ΔE figure can't fully capture. The hue will be bang on; the *texture* of the colour in low light may differ a touch.
This is a grey that lives and dies by light. In a north-facing room it'll go properly moody and atmospheric — gorgeous on joinery, a panelled hallway, or a front door. In bright south light it lifts and shows more of its green. Always test both on a board, move it round the room across the day, and view it next to your skirting and floor before you commit.
If budget's the driver, Dulux Smokey Slate in their Diamond Matt will give you a hard-wearing, washable finish at a fraction of the F&B price. Order a sample pot and paint two coats — never judge a deep grey from a fan deck.