Buying a Cot Mattress = Confused!

I've been putting off buying a mattress for our cot for a couple of weeks now as it seems there is so much you need to know. I feel it is such an important purchase that can not just be made at the drop of a hat. I am very conscious of the safety aspects involved with cots and feel a bit lost with it all. I sat down to research mattresses the other day and stumbled across this amazing website which cleared up lots of questions I had. 

http://www.babycentre.co.uk/a559747/buying-a-mattress

It talks about the different kinds of mattresses, mattress safety, materials used and pros & cons of each kind. It states that mattresses should;

• Be firm not soft
• Fit snugly without gaps
• Not sag
• Be between 8cm-10cm thick
• Conform to BS 1877-10:1997

We have a cot that fits a continental sized mattress (120 x 60cm) and is not a cot bed. According to the website the best mattress for us would be either a foam or coil spring mattress. I have been looking at prices and I am amazed at how cheap you can get foam mattress for (around £20). 

I still feel lost about which mattress to buy. Can you help? Do you have any recommendations? Which cot mattress did you buy? Please help!