Neil,
There are two main ways of controlling a modern “packaged” domestic boiler in the UK.
The first is the simple on/off switch or providing a power feed to the boiler as the “call for heat”. Most systems are operated this way. No matter how sophisticated or gimmicky the controller is - it merely switches the boiler on or off. Modulation of the boiler et al is all done with the internal control system of the “package”.
The second is an industry standard called Open Term. In this, internal control of the boilers’operating parameters is undertaken externally to the boiler by the thermostat / timer control.
Vallaint, refer to this as Ebus - which is in essence modified Open Term. The issue ( in my view) with Open Term is that it is not yet fully compatible across all manufacturers / suppliers.
What Valliant are proposing to you is to convert your control system to the latter. Unless you are very competent with control system logic, it is difficult to get a Valliant boiler to operate on Open Term unless you use their equipment. I did not think that the Ebus module that Valliant proposed you to use is available from them in the UK. I have always had to purchase them in Europe. I have also had an issue with them over a boiler warranty when a VR39 was used as the interface to control a boiler with their controls!!
If you opt for the first route, there are a number of non Valliant controllers that can be configured to suit your boiler - my preference would be Honeywell, both on grounds of cost and ease / flexibility if you want to upgrade to a more sophisticated controller ( such as Evohome in the future) or compatibility with a new boiler in the future.
Whichever route you follow, you are going to need a plumber / heating engineer familiar with the control system of Valliant boilers. Additionally, they need to be Gas Safe to make the connection / configuration within the boiler casing ( which is in the inside the sealed part of the boiler which makes the boiler sealed from the room).
If it was my call, I would get that person on board now. Their advice / recommendation will give you the most cost effective solution to resolve your issue.
With respect to Nest and Hive I would be wary of purchasing a supply and install package (circa £230) from someone who is not familliar with your boiler type
Hope this helps