I know OP specifically asked about 980, and while not really intending on turning this thread into another battlefield, but considering he mentioned
4K gaming for his grandson, isn't the £600 4GB GTX980 looks a bit draft (a bit is probably an understatement) when that budget can get two of these 8GB 290x and still with change in the pocket?
http://www.overclockers.co.uk/showproduct.php?prodid=GX-269-MS&groupid=701&catid=56&subcat=1752
If I recall correctly, the a single 980 isn't really that much faster than a single 290x, unlike at 1920 res. Though I guess it would be down to what PSU does his grandson's system has as well.