[VSTO] Help creating new worksheet from template

I'm currently moving an old vba addin over to a vb.net vsto com addin, it's all been going swimmingly but have hit a few snags.

Most pressing is this, when trying to create a new sheet from a .xltm template

Code:

---

Dim NativeWorkbook As Microsoft.Office.Interop.Excel.Workbook = Globals.ThisAddIn.Application.ActiveWorkbook
Dim vstoWorkbook As Microsoft.Office.Tools.Excel.Workbook = Globals.Factory.GetVstoObject(NativeWorkbook)
vstoWorkbook.Unprotect()
'create sheet, rename it and colour it:
Dim NewWorksheet As Excel.Worksheet
NewWorksheet = CType(vstoWorkbook.Worksheets.Add(,,, TemplatePath), Excel.Worksheet)

---

It works fine without the template, and I've double double checked the template path by debugging and navigating straight there.

Thanks for any help, you'd be helping a not-for-profit education establishment who are trying to (sort of) update their systems.