FWIW, there is no substitution for professional advice, the above system was installed by British Gas (BG).
but for reference and as a guide, I have a heat only system, i.e. hot water cylinder in the airing cupboard and water tanks in the roof space, flow temperatures to and from the condensing boiler are critical.
let me explain … (my thoughts not BG)
Firstly, stored water that is below 60°C may encourage the growth of Legionella bacteria (Legionnaires’ disease) therefore stored hot water should be maintained at a minimum temperature of 60°C in the cylinder to deter Legionella bacteria.
Secondly, in order for the Vaillant ecoFIT Pure to run optimally, the return water going back into the boiler should ideally be below 55°C (dew point is around 55 °) this encourages the condenser boiler to actually ensure optimal heat recovery.
As the hot water cylinder needs to be at a minimal stored water temperature of 60°C, the Vaillant ecoFIT Pure condenser boiler flow temperature should to be set around 65°C to ensure the water cylinder can actually reach a temperature of 60°C as a minimum and allowing for heat lost in the pipes (not radiators) before the hot water from the boiler reaches the hot water tank.
The challenge I see is that I can set the flow temperature of the boiler (water leaving the boiler) easily but to check the flow return water temperature requires you to go into installer mode via the control panel which is complicated and only available on the Vaillant ecoTEC models as far as I am aware 
Luckily, if you have a combi boiler, then the dilemma of stored water is not a problem and flow temperatures can be reduced ensuring a condenser boiler runs more efficiently.
Note!
Youtube clip showing Vaillant EcoTEC Checking Boiler Flow & Return Temperatures procedure
In a completely sane world, madness is the only freedom (J.G.Ballard).
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