I'm confused you saw it was available for your kindle so went to Waterstones and bought it in a non compatible format and now want to transfer it to your kindle, why would you do that ?
Transferring it will involve removing DRM and converting it to mobi, removing the DRM can not be discussed here. Either remove the DRM or get it from the darknet, your choice. Next time just buy it from amazon if you intend it to be used on your kindle.