Hoping someone can help me shed some light on the cause of the corrosion on this stretch of central heating pipe, and advise if it urgently needs replacing?
No, you've had a leak and that's the water and any additives/limescale that's stained/encrusted the pipe/fittings. If you take a green scourer and some soap and water you can scrub at that and clean it up. If you have any metal polish - say some brasso/t-cut on it it'll clean up easier.
A wire brush attachment on a battery gun would be even easier.
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