In heat loss for the window area it would be 20% better. But without other heat loss calcs to the remainder of the buildings fabric, would be difficult to put a figure on it.
1.2W/m2k is much better than what it used to be and perfectly fine. The max permitted is 1.6W/m2K but the building regs state 1.2 at the centre pane.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local