I’m new to this forum and wanted to share this solution with you.
I have just had the same problem as the original thread. That'll teach me!
I remembered from schoolboy chemistry that acid is opposite to alkaline so sponged out worst of water over solid caustic (just like concrete after 3 hours) and carefully poured on vinegar. No violent reaction but pipes did start to get hot. Vinegar turned milky. I was able to work out the softer soda, sponged it out (rubber gloves and goggles etc) and added more vinegar. After about half an hour I poured in rest of vinegar and within 5 minutes it was clear.
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