Advice re debt novation / potential winding up order

I have a situation regarding a client, who had novated a debt from a company he proposes to liquidate to a phoenix company. I'm having trouble getting the agreed debt paid and the client is going back on the novation despite me having an email to proove the novation / an independent witness of this

I've sent the debt to a debt collector and if this fails to resolve the issue i intend to issue a winding up order against the newco (the phoenix company)

Prior to this I want to take some advice on getting the winding up order