Honestly, pretty much any of the cheap aftermarket sensor kits will work well as long as you stick to the traditional "Near thing, make beep, repeat" type rather than the ones with the fancy displays - it's not rocket science, they're simple little devices and as long as you don't mind the drilling holes in the bumper part, can't go too far wrong. I've fitted all manner of cheap eBay kits for friends, and all of them work every bit as well as the OE systems as long as you get the mounting right. Occasionally better! You can even order the sensors pre-sprayed now to match the car from some places.
Worth checking to see if there's an OEM version available for the Astra though, if you want to keep it all Vauxhall and care about finicky people at resale time.