Citroen Xsara Picasso cannot put fuel in & fuel gauge gives incorrect reading

Hi there

I have the most annoying problem - I cannot put fuel into my car, as it keeps locking off the nozzle of every petrol pump I visit. Also, the fuel gauge is behaving erratically :(

I thought maybe there was a blockage in the tank vent pipe, but apparently not; I tried blowing out the vent with an air hose but that had no effect.

I've always kept the tank topped-up, and about a week ago I put 20 litres of fuel in which should have given me a 'full tank' reading on the gauge, but instead it was reading 3/4 full. No big deal I thought - maybe the float's got stuck. I drove the car for a couple of days and seeing that the gauge had dropped a little I tried to re-fuel. About an egg cup full of petrol went in and the pump locked off. I tried angling the nozzle in every direction possible but had no luck. I tried a different pump and the same thing happened.

Took the car to my local garage but they failed to work out what the problem is (they're suggesting taking the fuel tank off, if it's not possible to dismantle the filler neck with the tank in place).

I tried re-fueling again yesterday, and the problem persists - I only managed to get about 1 litre of petrol in before it just back-washed down the side of the car. When I started the engine, the fuel gauge was reading full (not true - I had a half a tank of fuel at that point) and that's how it's stayed.

I'd be really grateful if anyone can help!