Fuel Tank Leak


I have a V reg Terios. We have fuel leaking at the front edge of the tank. Our local garage removed the tank which they tested for leaks after we could smell petrol but could not find any.They reattached all pipes.

Any ideas as to possible cause?

A few months ago we lost fuel filler cap and have a non daihatsu replacement,.... would this have any bearing on it?

There is a small sump ontop of the tank that is filling up with petrol, is this to do with the venting?

I am totally non mechanical.

Update - Repair
We removed the fuel sender unit, cleaned up the fitting plate and the whole area where the sender unit attaches to the tank and comes into contact with the rubber gasket /o ring.We reseated the rubber gasket/o ring and applied a flexible silicon gasket around the outside of the rubber gasket(hopefully this actually should not come into contact with petrol if the o-ring does its job,but its all we could get at the time. This may need to be redone so if anyone knows of a good petrol resistant flexible sealer please post here.

The car is now not leaking at all.

Sounds more like a leaky

Sounds more like a leaky pipe rather than the fuel tank, it may be a pin hole or a bad connection which will let fuel sit on top of the tank, rather than a tank leakage.

i dont know anything about

i dont know anything about the pipe layout on a terrios , but it sounds like a leak somewhere above the tank itself, on the filler pipe maybe for the fuel to leak down onto the tank.. does it only happen on a full tank?... try not filling above half for a while see if that narrows it down. unusual to have a puddle of petrol it normally evaporates as fast as it leaks

Access to fuel tank pipes

Thanks for reply posts.. how do i easily access the pipes on top of the tank?

The leak occurs when both the tank is low or full..