Yes I too am a little worried, because the quality of installation doesn't look like it was done by a person with an unvented certificate. I am also conscerned that I can't clearly see the tundish.

The item to the left with a large black knob on is your Multifunction control valve, this has a number of duties, it prevents back flow to the water mains, filters incoming water and sets a maximum pressure. To the right of that a much smaller device usually in blue with a number 8 on the front is your expansion valve. In Europe it is normal to allow for expansion of the water as it heats to simply eject from the expansion valve. In the water conservation conscious UK we allow for expansion with either a bubble top or an expansion vessel, so any water coming out of the expansion valve indicates a fualt. Deepending how and when there are different causes behind that fualt. The output pipe of this valve is called D1 and it should fall towards a visible break in the pipework where you could view the water rushing through (or just dripping) this visible thing is called a tundish. The connection at the top right of your cylinder is called a temperature and pressure relief valve. If the user thermostat and also the overheat thermostat fail your next line of defence is the tpr valve. The discharge pipe d1 from this should also drop to the tundish. It is OK for the two d1's to join before the tundish. the discharge pipe after the tundish d2 has be one pipe size larger than d1, and must drop straight down 300mm before the first bend, then gradually all the way to the exit. The size of d1 and d2 depend on distances and have to be calculated. The position of the tundish should be such that you can tell what is going on, but that you won't get scalded by any discharge, it is acceptable to put them to the side of the cylinder not right in your face, but they shouldn't be hidden behind it. Your tundish could well be behind that tag on the right.

There is a zone valve for the central heating connection to the cylinder, which is correct. This fails shut in the evnt of a power cut which is the right thing.

I won't carry on about the position of the discharge pipe and so on, it gets more involved.

The bubble in this type of cylinder naturally becomes smaller until it is ineffective, but it is very easy to reform. The instructions are on the side of the tank. In a nutshell, shut of the cold water supply, switch off all forms of heating, open the nearest hot tap and hold open manually the tpr valve until all gurgling stops, this could take some time. Then let go the tpr valve, shut the tap and turn on the water supply. The bubble will naturally form.

IF YOU EVER SENSE ANY KIND OF DANGER you must A/ turn off all heating sources, i.e. central heating and or immersion heater/s. B/ Open a hot tap. C/ turn off the cold supply to the cylinder. Everything will settle down after that, get an expert in.

When water is under pressure the boiling point is much higher. If any part of the integrity of that vessel or directly connected controls should fail water under pressure would on changing to atmospheric pressure flash over to steam. There is enough power in that flash over from water to steam to blow your house out of the ground.

This particular cylinder is extremely safe and ruggedly constructed, all the necessary controls are designed into the fitting kit to prevent such an event occuring. But you should know it is not something to take lightly. I would recommend you have it checked by a qualified person.