Newbie Question - Mains transformers for PSUs

This is a very simple question.
I have figured out (with the help of this forum) what a mains transformer does (many thanks)
Just one question..
A lot of the PSU designs ask for a stepped down AC voltage ... eg. 15V AC.
Now I have seen the difference on the Group DIY page between a circuit board mounted transformer and a chassis mounted transformer.

Question 1 - does quality come into this or is it just supplying power and the sound comes from the main circuit.

For the UK market (230v) has anybody got any links ont he different types of transformers we should be looking at.
Are these any good?
http://www.clairtronic.com/special.html

And lastly - how do you wire them up to a AC socket with three prongs.
I have figured it out how to get from the socket to the wall (I do have some brains in there ... kettle lead isn't it?)

unless there are cost constraints, a torroidal transformer will give the smallest hum onto everything else in the box.
You haven't stated what you're building but the majority of gdiy projects use a split rail supply - that means the two secondaries of the transfomer (both say 15VAC) are used to make the plus and minus 15V needed to run most chips etc used in the designs.
You need to think about the power requirements of whatever you're building and maybe buy something slightly heftier - unless you're following a parts list. Is this for your box of tricks??
The link you gave is for amazingly cheap transformers, but are they pcb mount ? I suggest going to farnell and look around.
Most transformers have clear markings as to where to connect the live and neutral mains wires (after the units on switch and fuse)