Ah ha! Adobe have released an update to Lightroom. This is now up to version 4.3 and it fixes the speed issue I have been experiencing.Now, when editing an image within the Develop module the updates and previews are so much faster! Even the large scale panoramic shots which are over 10k pixels in size now only take around a second to come in.
Haven't had a good play around with it yet as I have only just installed it but after my Saturday morning (bloody freezing shoot) I will put it through its paces. See if the work flow speed has increased.
You can get the update from here.