Andrew thanks for posting this comment, interesting on three counts:
1 Who decides RHS advice? It’s dangerous because I suspect that not everyone in the RHS would agree with that, yet the word is as you phrase it, “the RHS recommended”.
2 I certainly don’t agree because we are talking compost here, “rotted manure” as you call it. Nutrients become non-soluble in water, almost all. If that were not the case, your 30 years of autumn-spreading would have resulted in a miserable garden. Likewise Homeacres would look washed out by now, 11 months since I added anything at all to the soil!
3 The vagueness of the ‘manure’ word. Is your alpaca manure fresh? If so, stack it co compost. Is it aged and dark, a bit crumbly and little odour? If so, it’s compost and spread it. This is why I changed the topic title to compost rather than manure. There is more clarity with ‘compost’.