Vauxhall Astra high revs on tick over

is the aftermarket one that you purchased a soldering one or the plug in type if its the solder one make sure you have the wires soldered correctly as this can affect the tick over but if you have a plug in one it could be faulty even though its new so try getting a replacement if you can and give that a go but the problem could be something else like a build up of dirt in the throttle body ie grease/tar like substance if so give it a clean with some carb cleaner