It turns out that Raynox actual sell a number of different macro lenses that can be used with the Sony DSC-F828. Given a slightly more limited budget, than when I bought the telephoto adaptor, I opted for the Raynox DCR-250. Raynox describe this macro lens as providing 2.4x magnification in comparison to my camera's normal macro function. On another page it uses a different metric and describes the magnification as 8 diopter. I'm not sure if these are equivalent or not especially as I seem to get much more than 2.4x magnification using the lens. Unlike the previous lens I bought this one doesn't simply screw onto the end of the existing lens. The lens is only about 4.5cm wide, much smaller than the lens on the camera, and is attached by screwing it into a lens cap which you then place onto the camera. This of course makes it easy to use with any lens to which the cap will fit (the spring loaded mechanism means it will fit lenses with a range of diameters). Attaching the lens to the camera is the easy bit. Taking good photos with it is much more difficult.
The macro lens is designed to work at the maximum zoom level of the camera (so x7 or x14 if I use the digital zoom as well) otherwise you get heavy vignetting around the edge of the image -- in fact I didn't zoom in fully on the buttercup in the photo for this post and you can see the darkening in the corners. The main difficulty though is that the macro lens gives a very shallow depth of field. You can see this in the photo as well. The stamens are in focus but the middle of the flower isn't. This means even the slightest movement and focus can be lost. It also means that the auto-focus on the camera is practically useless. The best way of using the lens is to switch to manual focusing and then focus by moving the camera towards (or away from) the subject until it comes into focus -- not an easy option when the wind is blowing the flowers around just a little too much!
I'm certainly finding this lens much more difficult to use than anything I've tried before but I'm hoping that with some practice I'll start to get the hang of it and produce some interesting photos. There are plenty of photos on Flickr that use the lens so hopefully I'll get there in the end. Just to prove the buttercup photo wasn't a complete fluke and that I did manage to get some other photos in focus here are just a few more from my experimental trip around the garden.