Helicopter; Seams ?

There should be panel lines present where the sponsons meet the fuselage.  Usually, just liquid glue will seal up the joint enough to represent the panel line well enough and you won't need to use an additional filler.

Like Goldie, I don't know too much about those things that just pretend to fly, but this pic shows a really smooth underside.

If anything, there's a break where the sponsons attach to the tub of the helo, But the geometry of the model looks to have those be kind of flush across the bottom, where the actual one there; they kind of stick out of the sides like wings.

I think it's just not a well designed area of your kit.

I'd eliminate all of the gaps/ seams.

Know nothing about the Lynx, but would assume smooth skin, maybe panel lines.  The smaller of the two you can probably fill and sand with putty.

The big gaps might have to glue in thin styrene strips, or heat and stretch some sprue frame to the appropriate size and glue in.  When good and dry trim with a knife and sand flush.  

Then rescribe any panel lines.