is horse manure good for clay gardens

Well it can be, provided its at least 2 years old and has been composted well with straw; not to be used fresh.

But you can also use horticultural grade sharp grit, spent mushroom compost, composted chicken manure, compost from your local council recycling centre if they sell it, organic soil conditioning compost from the garden centre, any of these things help if you dig them in.

Hi Trevor131...a big welcome to GoY. I garden on clay soil and it is the most fertile of soil, once you have it under control....Bamboo has listd the mediums you can dig in...but you also need a lot of patience. I would warn you that although privet makes a good hedge, it also takes lots of moisture and nutrients from the soil, so don't put your plants too near it.
Good Luck!!

Hi Trevor and welcome a word of warning about your proposed pivet hedge. Privet really is a gross feeder and will take a lot of the nutriment from your soil and, much as you cut it, will over the years take up a good metre in your garden.