Update:
This is not needed for Leopard!! The W300i will work natively out of the box with 10.5
So the latest version of iSync changed the way that plugins worked which meant that Miche was without the ability to sync her phone with her mac… this will be easy to fix says I…
After playing around for an hour trying to re-write the plugin myself and getting no-where fast I found this page which had a tutorial on creating iSync 2.3 plugins:
The site was incredibly helpful and with a bit of messing about I was able to get the correct settings for her phone and it all works again. Here is the fruits of my labours incase anyone else has a W300i that they would like to sync with their mac.
You need to extract this to:
/Library/PhonePlugins (then restart iSync)
The image is incorrect for the phone but is the nearest image I could find for the particular model (which turned out to be a SE Z520).
I had found websites selling their premade plugins but Im loathed to pay for something I think should be free, which is why Im sharing the above to hopefully save others the hassle!