You're in luck here, mate — this is about as clean a crossover as you'll ever get between the two Dulux ranges.
Dulux Alpine View is the match. It comes in at LRV 4.2 with a measured ΔE of 0 from Dulux Heritage Mallard Green, meaning the difference is imperceptible. Stand the two chips side by side under daylight and you genuinely won't tell them apart. For all practical purposes it's the same deep teal-green, just sitting in the standard Dulux colour range rather than the Heritage line.
If for some reason Alpine View isn't to hand, Dulux Heathland (LRV 3.8, ΔE 2.1) is the next best thing. That's still well within the "very close" band — under ΔE 2.5 you'd struggle to call it out on a finished wall, though across a large flat expanse in raking light a keen eye might just notice it runs a touch deeper and a shade cooler. For a feature wall or a small room, it's perfectly serviceable.
The obvious question: why bother matching across ranges at all? Two reasons. First, the standard Dulux range often gives you more finish options and easier availability through trade counters. Second, if you're topping up a job originally specced in Heritage and the can's run dry, knowing Alpine View is identical saves you a reorder.
One thing to flag — Mallard Green sits very low on the LRV scale (these are all under 5), so it drinks light. In a north-facing room it'll read almost black-green. Make sure you want that drama before you commit. Two coats over a grey primer like a tinted undercoat will give you the depth without patchiness; going straight over white tends to leave the first coat looking thin and streaky.
Order a tester pot, paint a decent A4 patch, and live with it across a full day before you buy the lot.