@rdipardo Thanks for a detailed reply. I thought that might be the case.
Having looked at it more closely, (lets call it the thumb, though I really like pea too!) - The thumb goes a little brighter when the mouse is over it, and it goes even more brighter when the thumb is grabbed. I wonder if the control can be customised in a future version, just to make it brighter before it is clicked. On a very large file, the thumb becomes very small, and I think the issue for me is that the default thumb colour is the same as the main text background. If the room is bright (with natural light, which I know is unusual for a programmer), then it can be hard to see it. But I would stress this is a minor issue and over all the product is fantastic.
I suppose a work around for now, is to try to find it with the mouse, now I know that it will go brighter when the mouse is over it. Also of course, holding down the LMB on the scroll bar, will eventually bring the thumb under the mouse. Thanks again.