OP, I have a Vaillant ecoTEC 418, VR65, VR10 & VRT392 and probably know more about how they work than Vaillant's UK support & many RGI's! 
A few points which may help you;
1) The Vaillant controls VRC470, 430 or VRT392 provide modulated flow control of the water temperature, independently for HW and CH so the boiler/controls will use a different temp to heat the hot water than heat the rads.
2) On the CH side, the controls attempt to keep a stable room temp by modulating the water flow temp provided to the rads. When the house is up to temp, the rads are often run luke warm, however when there is a big delta between inside/outside temp (i.e. 1st thing in morning / late evening on a cold winters day), the rads will run much hotter to heat or keep the room temp stable.
I would suggest you try balancing your radiators to see if you can increase the delta between flow/return at the boiler to 20C. In order for the boiler to condense, the return temp has to be below 55C, the flow temp is likely to be above 65C on a cold winters day so with only a 10C delta, the boiler won't run in condensing mode. Increase that to (as near as possible) 20C (via balancing / pump speed) and you increase the chances of always running in more efficient condensing mode.
3) Currently your boiler will modulate the burner / gas usage used to heat the water. e.g. your boiler will currently modulate from 25kW down to 12.7kW. It uses the flow/return water temp to work how much to modulate up/down as needed. With the addition of the Vaillant contols, modulation of the water flow temperature also occurs.
4) On the 400 series boilers (which can modulate down much lower, to around 5-6kW), there can be cycling issues when using the VRC430 weather compensation control (see here). I don't know if the newer 470 has fixed the cycling issue(s) or the same happens with a 6xx series but I would be surprised if there wasn't some cycling when there is a mild weather outside! (The control will be requesting low flow temps e.g. 40C and the boilers minimum 12.7kW is likely to fufill that too/very quickly causing the boiler to cycle!)
5) If your whole house calc is 17kW, I would try lowering d.1. from 25kW down to 17 or perhaps 20 and monitor your gas usage on a daily/weekly basis to see how it compares.
6) How many kWh is "£1250pa"? i.e. Gas costs can vary a lot so quoting kWh will give a better indication.
7) If you are from a engineering background, you may find my thread here an interesting read, it goes into technical detail about balancing and setting up the boiler/controls.
Hope this helps!?
A few points which may help you;
1) The Vaillant controls VRC470, 430 or VRT392 provide modulated flow control of the water temperature, independently for HW and CH so the boiler/controls will use a different temp to heat the hot water than heat the rads.
2) On the CH side, the controls attempt to keep a stable room temp by modulating the water flow temp provided to the rads. When the house is up to temp, the rads are often run luke warm, however when there is a big delta between inside/outside temp (i.e. 1st thing in morning / late evening on a cold winters day), the rads will run much hotter to heat or keep the room temp stable.
I would suggest you try balancing your radiators to see if you can increase the delta between flow/return at the boiler to 20C. In order for the boiler to condense, the return temp has to be below 55C, the flow temp is likely to be above 65C on a cold winters day so with only a 10C delta, the boiler won't run in condensing mode. Increase that to (as near as possible) 20C (via balancing / pump speed) and you increase the chances of always running in more efficient condensing mode.
3) Currently your boiler will modulate the burner / gas usage used to heat the water. e.g. your boiler will currently modulate from 25kW down to 12.7kW. It uses the flow/return water temp to work how much to modulate up/down as needed. With the addition of the Vaillant contols, modulation of the water flow temperature also occurs.
4) On the 400 series boilers (which can modulate down much lower, to around 5-6kW), there can be cycling issues when using the VRC430 weather compensation control (see here). I don't know if the newer 470 has fixed the cycling issue(s) or the same happens with a 6xx series but I would be surprised if there wasn't some cycling when there is a mild weather outside! (The control will be requesting low flow temps e.g. 40C and the boilers minimum 12.7kW is likely to fufill that too/very quickly causing the boiler to cycle!)
5) If your whole house calc is 17kW, I would try lowering d.1. from 25kW down to 17 or perhaps 20 and monitor your gas usage on a daily/weekly basis to see how it compares.
6) How many kWh is "£1250pa"? i.e. Gas costs can vary a lot so quoting kWh will give a better indication.
7) If you are from a engineering background, you may find my thread here an interesting read, it goes into technical detail about balancing and setting up the boiler/controls.
Hope this helps!?
