How to stop self closing doors from slamming
Beware of UK fire codes before replacing that closer -- which you probably don't need to do.  


Almost all of the closers of that type are adjustable, see below.
Rubber door bumpers are very cheap, can help a lot (but aren't as good as a well-adjusted, top-line, hydraulic closer), and come in a variety of sizes and styles.  Here's some at Amazon that should work well. 


Most of those chain tensioners are adjustable.  Turn the anchor plate widdershins to lower the tension. Just use CAUTION as the chain exerts considerable force. See below.



General procedure (defer to manufacturer's instructions if possible):


Open the door fully and block it in place.
Clip or pin the chain to keep it from retracting.  All closers come with a clip for this purpose, but sometimes you can get a small nail though the chain.  Vicegrips might work in a pinch. USE CAUTION.
Move the door slightly towards closed; the chain should go slack.
Unscrew the anchor plate from the door.  If it looks like the chain will retract, STOP!
Turn the anchor plate counter-clockwise to decrease the tension.  One half-turn at a time.
Refasten the anchor plate and unpin the chain.
Make sure the door closes as required by your local regulations.
Repeat as necessary.
If the anchor plate should become fully retracted while it is loose, it can be pulled back out using a claw hammer.