How much head of water to run a sprinkler irrigation system for my garden

This is a pretty technical question, Burchp.
If you are using a pump, and I think that you would need to, you don't need any head of water as the pump will do that for you.
I would suggest that you go to a Plumbers Merchants, explain your proposal and ask them what size of pump you need to give the same pressure as you would get off the public water supply to your house.

Most households are under 3bar pressure otherwise water will burst everywhere at the sink!

Are you certain you want to go that way? Plants don't want pressure (damages) they want volume. However-

From the pump start with 40mm pipe and don't go below 20mm. My set up can discharge around 18,000 litres/hour at 6m/head which makes a nonsense of a B&Q water butt!!

Well!! I am just trying to get a feel for what sized pump to run a number of sprinklers and sprays. Probably three sprinklers for the lawn and unknown as yet number of spays in the borders. Obviously the greater the pressure the more effective the sprinklers and sprays are. I don't think I need the capacity you have - probably 70litres at 3 bar will suffice. Thanks for your thoughts. Paul