Renault Kangoo 06 Erratic Fuel Gauge

[sunnym]

Hello People!
This is my first post and looking for answers/advice about my Renault Kangoo 06 which I have only had a few weeks.
I apologise for the long post in advance.

Here's the problem

First time driving Digital fuel gauge was showing 4 bars on digital display so just under half full.
Did just under 5 miles. Stopped and switched engine off.(had some new wiper blades fitted at Halfords)!!
Start engine again and fuel gauge is showing one bar short of being full...so has gone up but not whilst driving.
Drove 5miles and car ran out of fuel.
Obviously because fuel gauge was showing initially half then full it was not initially apparent to me that it was out of fuel
Having only done 10 miles on what looked like half a tank before level shot up to near full.

I put 35litres petrol in the car and digital display shows 2bars short of full and seems to go down to half full.
Car was driven by someone else for a few days and gauge seemed to go down slow and steady(about 30 miles)
I also reset the miles to empty display and this mileage went up from 77miles to 98 miles over space of 2 10 mile trips.
Also reset MPG display showing average 12mpg

Went to car wash and digital fuel gauge has gone down to half full. Switch off engine then restart and digital fuel gauge has shot up to one bar short of full again!

Put 20litres of fuel in car then...
Reset all displays and miles to empty showed 340miles....MPG display showed 34MPG! So it has done some sort of calibration but not sure if this is correct.

It has a smaller tank because it is a wheelchair access vehicle but I spoke to the people who converted it and they said that although some of the earlier model tanks could cause incorrect readings....the readings were not erratic as I have described.

Has anyone had this type of problem with erratic displays Is it a common fault? Any advice would really be appreciated!

SunnyM