I had front and rear parking sensors retrofitted by the dealer (Sytner Haroldwood), and all in all paid £500 for the pleasure.
They've been nothing but a pain. They do work, but I had shoddy workmanship and noticed a cable hanging out from above the exhaust so took it back for them to fix it, they did it. The other issue I had was that they would go off when driving around on their own, with nothing behind or in front of me. I noticed this was only happening when it rained. I took it back to them, and they said it was meant to only be activated when above a certain MPH threshold and wasn't doing so - they claimed they replaced the unit. A week later, it still goes off when stationary (e.g. at traffic lights) if it has just rained - so I'm going to have to take it back AGAIN!