Hi there.With it being a natural pond,the best way of hiding the liner is to plant trailing plants round the edge that don't mind being in the water itself.You could even put some more grass around the edge so it grows over.You do need to plant well behind the pond on at least 2 sides,so it helps shade the pond and gives amphibians somewhere to shelter when they first come out from the pond.In the spring through to autumn I cannot get behind mine at all.Ferns and hostas look very natural.There are many plants that like the damp,you could even make a boggy area as well.It will mean digging back uop some of the grass you have now but it would be wiorth it.
The pond looks great by the way,but it takes a while to mature so don't worry that it will look a bit sparse for a while. You could also put a pile of rotting logs in the middle of the plants that will encourage the insects that frogs and newts need to live on.
