So I've had my Fabia 1.9TDI for around two months now and always through it odd not for it to have ESP.
On further research I discovered that apparently it wasn't offered on UK models for some reason.

How hard would it be to retrofit ESP and does anyone have any experience in doing this?

As far as I know, the only UK pre-facelift Fabia mk2 that had ESP as standard was the 1.6 Auto. 

 

You may be able to find all the bits and wiring on a scrapped car. You'd need to change the ABS pump along with adding the additional sensors that ESP relies on.

Whether the ABS pump from a petrol is compatible with your diesel I don't know.

 

One other option to "rob" the parts from is the pre-facelift Roomster Scout. They had ESP as standard, and were available with the same 1.9 TDI engine as yours which may make things easier.

 

http://www.ebay.co.uk/itm/2007-Skoda-Roomster-1-9-TDI-PD-Scout-5dr-/391592994117