'Screwball' is putting it mildly. This is the nutter who was disowned by Corbyn & McDonnell as, to quote (McDonnell): "He is not the economic adviser and never has been, because we doubted his judgment"
As to his knowledge of tax issues, the best I can find is his advice to Guardian readers on how to avoid tax on their employment of domestic servants:
https://www.theguardian.com/money/2001/jan/14/childcare.observercashsection
What's the Stu Peters' connection?