All done - thought a couple of tips might be worth recounting. The fitting kit someone here pointed up, £10 on Amazon, is a cheap way to get the antenna 'booster' and a fascia surround. But it also includes a wiring loom that's completely useless. I fitted a relatively basic Pioneer unit and that came with the necessary adapter harness to connect straight to the Fabia loom. The loom in the kit is simply a pin-for-pin extension, terminates in exactly what the Fabia's loom does, and would give you another 20cm of leads and two connectors to stuff in behind the unit in the dash! Gave me quite an extended "Is it me, or..." moment.
Also the fascia surround doesn't slope with the dash, so leaves a small part of the top of the cubby hole exposed.
The Pioneer adapter has a blue antenna power lead which ends in an odd (to me) tiny clip. The antenna booster lead had a bullet end, so both were removed and a joint made. The P adapter also has bullet connectors in the red and yellow leads, so the VAG swap was easy to make.
The fit was extremely tight, good thing as far as rattles etc are concerned, but it does require an amount of faith and shoving. Fortunately sharp metal cage beats softish plastic dash structure, useful, in fact, that the edge of the cage is so sharp. My first reaction was that it would never fit, but the trick is to distort the cage very slightly by lifting one side a mm or two. This makes it a tiny bit narrower, and you can re-straighten it once it's some way into the opening.
With hindsight, I see that the antenna booster alone is available for under four pounds, so it might be worth checking if the Skoda fascia surround has the correct slope and, if so, how much it costs. If it's one of their saner-priced items, it would probably be the better route.