Well, I only thought I fixed this. I improved fps in general - but I can still get this problem. After spending all morning isolating and testing entities I now understand a little better what is happening in my level.

The trees and bushes are models - but the branches are 2d planes. If I put one of these trees or bushes in a level by themselves and aim the camera straight at them the refresh times are really high. If you scale these models to a larger size - the refresh time increases what looks like proportionally. So even though these models are low polys - they demand a longer refresh time. (I am guessing it is because some of these planes are viewed edgewise).

Having said that, I don't know why they behave as well as they do in the larger level - because I have lots of these bushes and trees but its only sometimes, at some angle, that these models will drive up the refresh time.

In the shot above - I believe the culprit is the tree on the upper LHS of the screen shot - which is scaled quite large. When I removed it from the scene - I had a more difficult time reproducing the higher refresh time. However because of the pink flowered bush in the foreground (also scaled larger)- I can still get the same results standing in certain spots and viewing the scene from certain angles.

The effects on fps are significant but may well just be the price of having bushes and trees...and still I am getting reasonable fps...so I think I will just have to accept that there probably is no way to optimise this other than to be careful about the scale as it does exaggerate the problem.