Nissan Note 2015 Premium Acenta 1.2 Reversing sensor Operation problems

[Music to my ears]

Hi i'm a new member bought a secondhand 40K Nissan Premium Acenta 1.2 found never had a sensor on a car before but found when reversing very slowly form 20 ft towards a brick tall wall nothing happened a slightly just audible quiet alarm then quiet. Is it very expensive to go to a local dealer to get this repaired if refused under 3 month basic used car warranty i am slightly disabled and useless and most DIY or is a main dealer Nissan best 100 plus miles away as i am in a remote Highland area Scotland within UK ?

Thanks 

[Trevor]

You can replace the sensors which may be the issue or if the sensor alarm is in the boot area, is it being muffled by something up against the boot sides ?