Mushroom compost, horse and chiken manure are all good..and although they all provide nutrients for plant growth, they provide very different quantities/strenght if compaired to each other.
There is nothing wrong with horse manure and if you have easy access to it..carry on, but on it own it maybe lacking with certain things that your friendly OAP's 'mixture' will provide.
Manure will do good for soil structure, but you may need to adjust the acidity of the soil with lime between crops and give some additional fertilizer.