Aftermarket alternator problems

[DuffJ02]

So I recently had my alternator changed on my X5 4.0d for an aftermarket part, although one which was listed as compatible. Since having it fitted, the car starts fine, but when it's left stationary for approx 20 seconds or so the "Charging error" warning pops up. As soon as you move, this goes off. I spoke to the indie who fitted the alternator and he said this was due to an issue on the smart charge interface with an aftermarket alternator, but I don't fancy paying the 1k for an OEM part. Has anyone else had similar issues?