This is your property. If an individual is holding on to your property without your permission then that is theft. Write to your ex-employee and remind them that the laptop is company property and provide them with copies of any documentation that you have that confirms this so it must be returned.
 
Set out clearly within the letter the discussions that you have had to date, including their refusal to return the property, and state that they are to return the laptop and other company property within their possession by a set date, normally seven days from the date of the letter. Also, state that this property must be in the same condition as when it was provided to them otherwise you will report the matter to the police for theft and will sue for any losses you incur. If it is not returned, or it is returned damaged, then go to the police and report the theft.
 
You do, of course, have the option of going to the police straight away, but they will normally prefer to see that you have made some attempt at resolution.

See also: £50 reward on using Electronic VAT Return