Anyone had issues with parking sensors knocked out?

Hello again!
Recently I have had a fault with my parking sensors, thought I'd ask here before contacting the dealer. A few months ago I had a message saying clean sensors, just as I passed a jet wash. Though the sensors didn't seem obscured, I washed them and it was fine.
A few days ago, same message. I stopped and cleaned light dirt off all, didn't sort it. The screen usually displaying hybrid functions is then just full of the check sensors message.
At work I cleaned all sensors thoroughly with water and tissue.
Still not fixed.
I washed the car completely, paying extra attention to all sensors. Still knocked out.
Over the last few days they have mainly been off, occasionally after turning a corner (loose connection?) they come back okay.
Or just starting up they may be working okay, or towards the end of a journey they start working again.
It's a pain losing the hybrid monitor, it's distracting the beeping and flashing as each time the sensors fail a message appears.
Anything I can do myself, or should I call my local Toyota dealers?

 

Thanks - the dealer is less than 10 miles away, but it makes me late for work if I have to drop by. As soon as the fault occurs again I'll call them (haven't used her today, the sensors started working again late yesterday).

 

Update - nothing found. I was pointed to the windscreen radar sensor (which I hadn't even considered), if this had been smeared it could have caused the fault apparently. Anyway, another Aygo for the day. They are fun and well loaded for such tiny cars.