The
Rawlplug site is down currently, but they have advice and specs of all their products. They'll have something to suit and are available from all the DIY sheds and builders providers.
With the insulation backed board, you won't be able to use the toggle fixings as these require a cavity. But you still have a few options, including, but not limited to:
1. Use a regular plug driled into the concrete and screws long enough to pass through the bracket, insulated board and into the plug. You could also use sleeve anchors in this manner, but these might be overkill here.
2. Glue a wooden batton to the wall using a decent construction adhesive. Use screws to hold this to the wall while the adhesive cures. Then attach the curtain brackets to this batton.
3. Use screw-in fittings such as http://www.amazon.co.uk/Rawlplug-Metal-Selfdrill-Plasterboard-Fixing/dp/B0001P0K8G (these).
Leo