Our piles of leaf mould (the black stuff) and manure come from Buckingham Palace gardens and the Royal Mews and arrive subject to availability. The manure is in fact stable bedding straw. Deliveries vary with the time of the year; the horses go to green field pastures for a rest when the Royal family leave London for extended periods. The RHS general introduction to manure is here.
Leaf Mould: People seem to prefer the leaf mould, so do read the RHS advice. Leaf mould has lots of fungal activity so is especially good for woody plants e.g. fruit trees and soft fruit bushes. It also improves soil structure. In 2011 Alys Fowler wrote a useful piece on the subject in The Guardian . And of course you can make your own leaf mould in the autumn with the deliveries of fallen leaves to the Southwark green waste holding area.
Manure: Horse dung is high in Nitrogen (N) content. However, the straw content locks up the N until it is well rotted so it is worth adding a N rich fertliser; National Growmore will work well. It is very good in the long term for soil structure. The manure from the Royal Mews is very fresh dung, but it comes with bedding straw which is relatively clean compared with other stables (such as the browner police manure we also used to receive). As well as straw, the manure contains some sawdust or wood shavings. It is excellent to add in layers to compost heaps. It is good for green crops if dug in during the autumn; you can use the leaf mould to create a seed bed. Be careful of the lumps of dung given they can burn plants. It is good to collect them and place them on a compost heap.
Health Concerns: Remember bedding straw manure contains horse faeces and is soaked in urine so do take care. Wear appropriate clothing, i.e. gloves and boots, and protect your legs. Wash your hands after manuring. Do ensure your tetanus injections are up-to-date. Parents please discourage children from playing on manure heaps, (bare footed children have been observed playing on the heaps in the past).
Tetanus is rare in the UK, but is very serious and can be fatal. Many will have been vaccinated during childhood and the NHS advice is to have a booster injection. If you are not sure whether you completed the full NHS childhood programme, the NHS advice on tetanus can be found here.
Pesticides in manure: there are concerns that manure and leaf mould can be contaminated with pesticides, but Buckingham Palace have a minimal pesticide policy and are working towards phasing them out entirely, more here.
Robert Holden (Gunsite Manure Monitor), 26 March 2020