New Car - Parking sensor problems

We've just sold the other half's Scooby privately & bought a new Mazda 5 2.0 Diesel Sport (got a cracking deal) to which we added front & rear sensors. Interestingly they gave us the choice of two...pukka Mazda ones (for about £600) or alternatively 'better' ones (in their opinion) fitted by local 'specialists' (I use the term loosely !) for just over £400...we took the 2nd option.

The car arrived last midweek & I drove it for the first time at the weekend (wife is currently signed off work, expecting twins soon !)...I immediately noticed the rear headlining was hanging down where they had fitted one of the displays. Then when I tried out the sensors, the front ones didn't beep at all & the display was erratic...it was very slow to respond. The back ones were even worse, they beeped when there was nothing there & continued to beep & display something at one side, until reverse was deselected.

I called Mazda & they said they would get in touch with the 'specialist', who duly called me to ask what the problem was...I explained & they said they would take a look if I brought the car down. I suggested I would do it on the way home after work, as I needed the car/didn't have time whilst working...that would still be a couple of hours before they closed...but they didn't seem too keen on that, despite saying, "oh it'll only take about an hour to fix" earlier in the call. They advised they would call me back...but didn't, so I just went down...and was meet with an excuse of " the workshop has been busy "...so what, I'm busy & have just spent £17k on a car which has another 400 quids worth of next to currently useless 'sensors' fitted to it.

Anyway, they looked at it - fixed the rooflining (just poppers not engaged, but wiring behind, so glad I didn't do it !) & checked out the sensors...they got the front ones working & suggested there is a delay, which is why I thought they were not working...sensors with a delay, that's helpful...they only kick in when you are within a metre & beep within 0.5m apparently, once the delay has passed ! As for the rears, one kept on activating falsely & not clearing, even when the object was moved. Some fiddling later, that seemed to have been resolved, until today...when the same thing has continued to happen again, as soon as reverse is selected, it 'detects' an object 0.3m away or goes straight to P (for Park or stop !)...this continues to be the case, until reverse is deselected.

I'll call them again in the morning (Mazda that is, as I paid them) & give them another chance to fix them...but I am thinking that faulty sensors are even more of a problem, than no sensors at all...and I am £400 out of pocket. Clearly they can't remove them, as there will be holes in the bumper (unless we fit another set of course)...but what would you do?

Apart from the sensor issue, we are delighted with the new car (now that I have the manual, parcel shelf & boot carpet...which they 'forgot' to include when the car was delivered !)...and I still have my Vrs to play with!

The sensors are Steelmate 800MN's??? I have googled the make & they seem fairly reputable...