Have checked both sets of instructions and the following is mentioned in the washing machine daily use guide,
“Contact our Technical Assistance Service to check whether the following accessories are available for this washing machine model. Stacking kit With this accessory you can secure the tumble dryer to the upper part of your washing machine to save space and facilitate loading and unloading of the tumble dryer”.
Not very helpful I know, as you say the only one is the WPro one at £70 odd part number C00378975, I would phone Hotpoint again and ask the question again using the part number just quoted.
I have seen and done the fitting of a shelf between the two appliances and a carefully placed strap to the back of the dryer and the wall behind so it doesn’t topple.
Don