Its in the middle of the tank on the top between the two inspection covers. It's glued in so probably not intended to be replaced easily.
Worth trying cleaning the sensor rods with a scouring pad through the inspection covers first.
These sensors aren't very good or accurate even when they work.
Yours will be a CBE sensor unit and they do fail.
You will find it on top of your water tank circular and white . It will have a reference number on it that must be used to order a replacement.
Marcle Liesure stock most if not all at around £30 plus delivery.
I replaced one recently for a FUN member and it was an immediate transformation
Every 12 litre watering can increased the reading by 10% so quite accurate and Karen was quite elated
No need for apologies matey
I had to pry out the white sealing gasket that you can see through the holes in the perimeter and run a 12" hacksaw blade right through with the blade in the 45 degree position in the frame. This left some white plastic debris inside the tank but not a major job to clean out. I fitted the new gauge hand tight, much easier to replace than remove.
I have found a small calibration screw on mine (under little green sticker ) put 25 litres in adjusted till guage read same now not to bad reading. Having said that its only the last 20 or so litres I am interested in for traveling between aires etc .
Interestingly our gauge is 13+ years old and has always been really accurate, until this last fortnight when it has decided that 60% is the new 100%. I am going to try and calibrate ours tomorrow (Sunday) all being well and will feedback accordingly.