No Fuel?!

[s88urd]

hey all, got a ph2 mk2 172, was running fine until earlier today.
I get in it, and the petrol guage doesnt move at all after firing it up.
Gradually the fuel gauge goes up a bit so i go and fill up with a full tank.
Driving off the forecourt and the same thing happens except the fuel gauge doesnt move at all.
Disconnected the battery for 10 mins and this seemed to sort it................for a while, was just driving and the fuel gauge gradullay went all the way down again.
Now the fuel gauge will not budge at all (it will move if i perform he diag test, although this says i have no litres of fuel!!, i have checked for a leak and disconnected and reconnected the connector under the back seat)
please can i have some help

 

[s88urd]

have just tried the Diag test again and it keeps coming up with a J error

 

[Storm_Force]

J means a fault has been noted on the fuel gauge. It must have disconnected for longer than 100 seconds. The resistance from the fuel tank sensor needs to be between 5 and 350 Ohms.

 

[s88urd]

saw that on the diag page, to be honest dont quite understands what it means.
is this the fuel gauge on the dash side or the fuel gauge on top on the fuel tank

 

[Josh@Renparts]

if it needs a new sender pump unit i have one in stock, 30 plus delivery.