I thought I would describe the two methods I have used for having a higher flow temperature for HW when using Evohome.

I have a Vaillant 438 boiler. I use Evohome and OT. In order to use OT on the Vaillant boiler, I use a VR33 - which is nothing but an OT to eBUS translator.

The Vaillant 438 is able to have two separate settings for CH and HW. However this is only available when using Vaillant's own eBUS controls and their wiring centre. When you use Evohome, a lot of that becomes redundant. The other issue is that Evohome has no way of telling the boiler that it's doing a CH or HW cycle, even though the OT protocol allows for this distinction, Honeywell/Evohome hasn't implemented that. Instead it simply raises the Target Flow temperature to constant 90C whenever HW is to be heated. The 90C flow requirement is deliberate and is actually relying on the boiler clamping that down to something more sensible. Luckily the Vaillant boiler's Flow temperature knob remains operational even when using OT via the VR33.

Solution 1:
My previous solution involved the following components. An HGI80, an eBUS monitor and Domoticz.
My boiler Flow temperature dial was set on max on the boiler. Using the HGI80 I would detect when Evohome was performing a HW reheat, that would trigger a script in Domoticz. The script used the eBUS monitor to issue a command that basically varied the "Maximum Heating Dial setting". So when Evohome was doing HW, I would set the max of that dial to 70C, but when it was only doing CH, I would reduce that max dial setting. This solution relied on the Vaillant ability to set and clamp the max flow temperature the boiler was heading towards.
This solution worked quite well for many years, but it relied on too many things.

Solution 2:
Using a VR65 wiring centre. The VR65 has many functions and allows for the pumps and valves etc to be all connected through it. But one of the features of the VR65 is that it can be used to detect when HW is required and that in turns makes the boiler behave differently.
Normally you would use an NTC sensor VR10 to detect the temperature in the HW cylinder. But with the Evohome you don't need that. Not having the NTC connected to the VR65 means that the Hot Water temperature dial on the boiler does nothing. The VR65 operates in a default HW priority mode. This means that while the cylinder thermostat is closed, the VR65 makes the boiler uses it's HW settings. If the cylinder thermostat is open, the VR65 assumes that the HW heat demand is fulfilled and then reverts to its CH heating mode.
Besides setting a higher target flow temperature (D.78), you can even set a different KW partial load (D.77) to use when in HW mode. This feature is brilliant. One of the issues with the Vaillant 438 and Evohome is that very quickly the boiler becomes over-sized for the house, as zones' heat demand start to get fulfilled. But you still need the higher load for HW. The VR65 allows for that. So for example, you can have a 18Kw boiler load (D.0) @ max 55C (D.71) for CH and then use 28Kw boiler load (D.77) @ max 70C (D.78) for HW. Ofcourse in CH mode the D.71 is actually the clamped maximum, and OT will often use lower target flow temperatures when in CH mode, visible in D.9

The trick to use the VR65 in this dual heat manner with Evohome is.
a. You can either send the same Switched Live from the DHW BDR91 to both the DHW zone valve and the Right hand terminal of the CYL terminals in the VR65. So that way, when Evohome calls for HW, it opens the DHW zone valve and makes the VR65/Boiler enter HW mode. The wiring is becomes very simple. You simply send the Brown wire of the DHW zone valve to the right hand CYL terminal as well.
b. The other alternative is to use the microswitch in the DHW zone valve to bridge the contacts of the CYL terminal. So you connect the Grey and Orange wires of the zone valve to the CYL terminals, as dry contacts, with no Live connected to either Grey or Orange. What happens then is when the Evohome DHW BDR91 makes the DHW valve open, the micro switch inside the DHW valve is closed and that completes the CYL circuit. This makes the VR65/Boiler enter HW mode. The advantage of this method is that unless the DHW valve opens, the boiler is not put into HW mode. Whereas in the previous option, if for any reason the DHW valve was stuck, the boiler is still kicked into HW mode.

No other connections are required inside the VR65 because you still want the zones valves, HW kit, schedules, weather compensation etc to be controlled by Evohome. You don't use any other thermostats on the eBUS in conjunction with the VR65 because they will then override the VR33 and stop Evohome from being able to control the firing and temperature of the boiler.

I hope this is useful to any one who has wanted to know how to use Evohome with dual heating mode.