A number of hotel rooms are available at guaranteed rates for SISPAD 2013 delegates - these can be booked here.
It's also worth checking directly with the Grand Central Hotel to see if they have any other rates available.
You are free to organise your own accommodation if you wish, particularly if you intend to extend your stay. Reviews of Glasgow hotels can be found on TripAdvisor, while Booking.com has a good selection of hotels available in Glasgow so you should be able to find something that suits your needs.
Below is a map on which we've marked a few hotels close to the conference venue which you may consider.