Does anyone know whether it is possible/practical/economical to retrofit ESP? Couldn't wait for a factory order Octavia...

I doubt it will be economical. Possible ? Maybe if the ECU is the same.

Not a clue how much would be charged, worth dropping your dealer a line perhaps, they may be able to find out for you

Audi spent hundreds of thousands of pounds shipping early Audi TT's back to Germany to have ESP retro fitted as part of a recall after handling concerns of early models as it wasnt viable for a dealer to do the work.

From what little knowledge I have of the system (that is, finding bits of it *wherever* I lift a piece of carpet) I'd have to say no.