i'd be worried if its a 2 yr city & guilds.
Indeed, nevertheless I guess we should attempt to help a lady in distress...
So, Igorbab, here goes
1) Almost. A bar mixer is one type of mixer shower. You should install thermostatic mixers with a combi boiler. If it's tank fed then you may install non-thermostatic mixers, however thermostatic ones are always the preferred option. Mixer showers should generally be installed in systems where the hot and cold water pressure is reasonably balanced.
2) A power shower is generally defined as an electric shower with an internal pump, as Burnerman has already alluded to, and these should only be connected to low pressure water systems (IE tank fed). Power showers draw from both the hot and cold water supplies.
3) A mixer shower is always fed by both hot and cold water, yes.
4) Confused question, but I'll answer as best I can. There are several different types of shower on the market, and these can be split roughly into two groups; manual and electric.
In the manual sector you have mixer showers, which can either be thermostatic or non-thermostatic. Thermostatic mixers compensate for changes in the incoming water flow/temperature and regulate them, giving a nice even temperature shower. Non-thermostatic mixers just slosh together what's supplied and spit it out the shower head with no regulation. There are also digital showers, which use electric controls but are essentially a form of thermostatic mixer shower, although some people may argue they come into the 'electric' category... Mixer showers may be fed by either low (tank) or high (mains) pressure hot and cold water systems. If they are tank-fed, however, a pump may be added to make them high instead of low pressure, increasing the flow additionally.
In the electric sector you have standard electric showers, which are fed by either a cold water storage cistern (CWSC) in the loft, or the mains, depending on the shower model. They are cold-feed only and heat the water internally. They may be thermostatic or non-thermostatic.
Then you have power showers, which as described before have an internal pump and are fed by both hot and cold water at low pressure. These may be thermostatic or non-thermostatic.
5) Pretty much already covered, but there are four types; electric, power, digital, and mixer. All may be either thermostatic or non-thermostatic, except digital showers which are always thermostatic.
6) Both power showers and mixer showers may be piped from the cylinder and CWSC, or you could take feeds for either off the bath feeds (this may well be debated in a minute by someone else on the power shower issue, but we won't go there for now). Electric showers may be taken from either the cold mains or from a feed from the CWSC (or indeed the cold on the bath tap), depending on the unit being installed. Digital showers run on either high or low pressure, depending on the model.
