Hi SteveSKLandscapes,
You would need to setup a new PAYE scheme for the LTD company,then you would make the employees leavers from the sole trader scheme and new starters on the LTD scheme.
You may also wish to look like TUPE, which would allow the employees to keep continuous employments between the 2.
You can find information on this by here:
Business transfers, takeovers and TUPE
Thank you.