I’m thinking of transferring to manual machine from a semi automatic machine. What are the differences between manual and semi-automatic machines? I'm interested in how difficult they are to use and in getting consistent results.
It depends greatly on the actual machine, e.g. when used correctly, a £1000 manual La Pavoni lever machine will by far produce a better espresso more consistently than a £60 semi-auto from your nearest department store.
There are two different types of semi-autos: those where you have to flick a switch on to start the pump and flick it off again to stop the pump (you can use a measuring glass to ensure you're yielding the same amount of espresso each time), and those that are 'volumetric', i.e. you program into the machine how much water you want it to dose, and it will dose this amount every time.
Manual machines (e.g. the La Pavoni Europiccola) require you to use your hand to push a lever that will in turn force water through the coffee. It's not as easy to get consistent results right away, as there are more factors involved that will very easily vary (the amount of force you apply, the time over which you apply it)
If you could provide more information, or share any particular machines you've been looking at, that would help us to build a better response that would answer your question a little more fully!