I fitted a replacement Instrument cluster (pre-owned) it would not be accepted. I re-fitted the original, only to find that the ECU 03 Brake electronics had lost it's Long Code. Would you be able to recover the original Long Code and instal it into the ECU ?

It just so happens I've been through this nightmare not long ago and buddy you're in for a headache.
You might have it stored on VCDS or OBD scanner from previous reads, in history. I think full scan on VCDS does also list details.
Option 2 is call stealership and talk to them about recovering code from VIN, but I do not know how this goes, and I'm certain it ain't gonna be cheap.
Option 3 is NOT recommended, to suffer with AI and keep asking correct answers VERY VERY meticulously and then triple or quadruple check what it gave you.

Part of the code is from VIN digits so you have to have it. Part of it is simply your car being Skoda Fabia 2011 but rest is gearbox, differential, traction control and you might brick those electronics.
It took me 4 hours. I bricked chinese vcds cable, and had to correct "final" code about 12 times with advanced LAUNCH scanner. Good news is, that said LAUNCH kept me safe from few incorrect codes and it had correct security code written into it.