I'm looking at changing the timing chain on our Fabia, and will be after some advice.

 

The car has aircon installed, so has an auxiliary belt like this: http://workshop-manuals.com/skoda/fabia-mk1/images/fabia-mk1-1795.png

 

How do I go about releasing the tension on that to get it off?

5 is the tensioner, it's spring loaded.  Follow it back and you'll see a chunk of metal with a round square egg shape hole in the end. You need a special tool to take tension off, but you might be able to use a 1/2 ratchet handle instead. or some grips on the arm itself.

 

Another way is to remove the two bolts holding the thing, which is easiest to get the belt off, but it's still a pain to get the belt back on.

Apologies for not getting back to you until now, thanks for the pointers, very much appreciated. When I get round to doing the job, I'll let you know how I did it.

I read on a Czech forum you need a tool which can be made from a bolt

 

if you have an M12 bolt, (bolt, not a screw, bolt has the non threaded part) i cut it down then using the non thread part file two flats, one on either side. So that you have shape similar to two D's back to back. the flats must have a thickness around 8.5mm 

 

so its essentially 12mmx8.5mm I made it to 12x8mm and it was very loose but still usable.