Well... finally picked it up at 4:30 this afternoon, was starting to sweat, as Warranty Direct were being a pain about authorising the work! Think they were confused because I got three years free servicing with the car and hence I had no Invoice for the services (sent direct to SEAT UK) though the service book was stamped up to date.
All appears OK now, warning light now goes out and volt meter reads 14V when engine starts.
Also... the heated rear window now works again. So my assumption that it won't work unless the alternator is charging the battery, appears correct. Something maybe worth taking note off.
I'm assuming they've fitted the correct one! It's a Valeo SG9B015 and from what I can find on the Internet, it looks to be a 90A one (as it should be). Though I did notice that the connector with the two small wires does not fit too well on the new alternator (IE. it does not "click" home on the connector). I can maybe see that working loose over time.
The garage would not tell me what the final bill was, I just had to pay the £25 excess under the Warranty cover. Though again from looking on the WEB, the only Price I could find for it was an almost UNBELIEVABLE £248! Good Good, are alternators really that expensive now.
All well the ends well I suppose.
Thanks for your input folks...