Fabia 1.9 SDi beep and battery symbol

Hi, before xmas my Fabia 1.9 SDi suffered a cambelt failure when driving on the motorway, wouldn't start and got towed to garage. Garage said engine was kaput and needed the works, they fixed it (though not sure what exactly they changed as the receipt didnt list everything they had done just titeled "engine work", I wasn't happy about that and asked for an itemised list but never received one, seems abit fishy but I dont want to waste my life getting upset about it) anyway car seemed to be running fine, though aux belt was looser than before and making a noise so took it back and they adjusted something (pully/pully bearing I think) when I got the car back the belt wasn't loose but it did/does make abit of a squeak sometimes when running, this has mostly gone now.

But my main problem is that when driving, particularly at high speed or when ever high or unstable revs such as shifting gears down or up from a low gear I get this beep and the battery symbol appears on the dash for a brief few seconds then dissapears until I do something like change down or come off the accelerator and again it beeps and the light comes on. Ive been ignoring this for afew months (as I was skint and had just spent all that money on the engine rebuild).

Im guessing it has something to do with the aux belt and alternator (as my book says the light means alternator not working properly as in not charging the battery, battery has been fine though). Or could it simply be a sensor that has gone abit daft and is thinking there is something wrong due to the vibration/speed of the engine at certain rev levels?

It never did this before the cambelt failure, and its really annoying me, I really want to keep this car after the investments ive put in to it, today I just had a geometry check and had all my wheels all re aligned so the steering is spot on and I want it to be perfect for the summer.

Thanks for any help in advance my fellow Briskodians.