I have recently gained an MPhil. How big a jump is it to convert the thesis to PhD?Does it normally take 2 years?

Have you actually written up and been awarded an Mphil? If so, I don't think there is an option to 'upgrade' to a PhD. You have to declare in your PhD thesis that you have not already been awarded a degree based on any of the work included in the thesis hence if you have been awarded an Mphil you would not be able to include the work that you were awarded the Mphil for in a thesis for a PhD.