These showers function the same way as a mixer tap, but they combine hot and cold water from respective taps into a single stream. This allows you to control the temperature of the stream.
Hot water taps often run off a hot water tank so if that tank drains completely, it will then need to be refilled before the shower can be used again.
However, some showers can run off a combi boiler if the mains pressure is high enough.

Fitting new taps might be required either because it is a part of a new bath/basin, or because the taps on the existing bath/basin have become old and worn, or simply for a cosmetic upgrade.
The type of tap you are allowed to have depends entirely on your bath/basin. If there is one hole for the taps, then you will need a mono-bloc mixer (these have two smaller diameter pipes coming out of the single hole for both hot and cold). If there are two holes, you can have either separate hot and cold taps, or a two hole mixer (two taps converging to the same spout).
Some mixers also have a lever on them for controlling the plug (such as opening and closing).

A condensing boiler is a boiler that has great efficiency, with an average efficiency of 82-29% in a real house installation (although, laboratory tests claim up to 95-98%) - compared to an average efficiency of a non-condensing boiler at 50-80%.
This increase in efficiency is possible due to the boiler re-using the heat of the flue gases and vapours to heat up water, which ultimately wastes less energy.
Flue gases typically reach temperatures of 180-200°C, meaning a lot of heat (and, therefore, energy) is wasted to the atmosphere. For this to work, the gases must be extracted into a vent, and for the excess water collected (the water that condenses to release the heat) to be drained away using the standard waste water pipes.
All new gas boilers installed since April 1^st 2005 (and April 1^st 2007 for oil-fired boilers) are required to be condensing boilers, unless exceptional circumstances apply.

There are many things that a boiler service must cover.
The location of the boiler must be one that allows for the correct operation of the appliance.
All fans must be in working order, and the ignition system must correctly initiate the burner.
All seals around the boiler must be verified as intact, and the gas connection point to the boiler needs to be free from gas leakage.
All surrounding pipework and the closing plate must be checked for integrity, and the heat must be confirmed to be passing heat to the water.
All controls (including the thermostat) must be tested, and all electrics in and leading to the boiler must be verified as being operationally sound.
A gas tightness test is also carried out to check for any gas leaks leading to the appliance.

However, even if the boiler passes the test, if there is a smell or detection of gas, this means the leak has to be traced regardless. Additionally, the boiler must also be run for a while to verify that the appliance is able to withstand extended operation durations.