Just to add something to this, the WD40 solution has been around for a while and it does work, as I have seen again this week.
Following all this rain I have been getting the long beep from my new to me D2 when engaging reverse for about a week now, I've waited for storm Dennis to pass. Sunday afternoon after the worst was over I gave each sensor a liberal spray and wiped it off.
I didn't use the car yesterday but today hey presto! Working sensors once more. Second time this has worked for me.
I will also mention that it gives excellent relief from bee stings too! Seriously, I have personal experience of this! And I'm not kidding either
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2002 D2 S8 - Avus Silver, Valcona red leather, Dark Myrtle wood inserts, Heated rear seats, Heated windscreen, Auto dim rear view mirror, Bose, Xenon Plus, Servotronic, Extended leather, Heat & UV Reflective glass & Ski hatch. Gearbox went bang but she's sleeping safely away.