Short answer: you don't need to pay Heritage prices for this one. Dulux Alpine View (LRV 4.2) is the same green at ΔE 0 from Mallard Green — that's not "close", that's imperceptible. Same brand, same factory, standard range, friendlier price. If you like Mallard Green, this is the swap, no compromise.
Why bother with Heritage at all then? The Heritage line gives you a slightly different sheen and finish character, and the colour comes pre-curated in that range. But if it's purely the *colour* you're after, Alpine View delivers it identically. Spend the saving on a better roller and a second coat instead.
If you fancy shopping around a bit, Crown Racing Green (LRV 6.2) comes in at ΔE 4.8 — that's a noticeable step away. It reads a touch lighter and slightly different in undertone, so treat it as "in the same family" rather than a match. COAT Brewer (LRV 7.2) is further still at ΔE 6.9 — a lovely deep green in its own right, but a genuinely different colour, not a substitute for Mallard Green.
The "but what about the finish?" question matters with a colour this dark. Greens this deep (all under LRV 8) drink light and show every imperfection, so prep is everything. Fill, sand, and on a previously glossy or patchy wall get a coat of grey or tinted primer down first — it stops you needing three or four topcoats to bury the old colour. Zinsser Cover Stain or a decent grey undercoat earns its keep here.
Practical tip: order a sample pot of Alpine View and brush it next to your current Mallard Green reference if you've got one. A ΔE of 0 means you'll struggle to tell them apart under the same light — which is exactly the point. Buy the cheaper tin with confidence.