Davros, you mentioned the Visual Time Tracker (available on the Letterbox site) a few weeks back, and I'm sending a thank you here as it has helped so much with my ds. The timer makes clear sounds and has a green, amber, red light system before time is up. My son finds it really difficult to move on from his books - which have always been his comfort/stimmy activity of choice. From the first time I used the timer it seemed to give him a very clear warning of change and helps us to have set boundaries over his use of books. Moreover, in his mind it is much easier to stop when the timer tells him to, than when I do. Would really recommend this to other MNs who are facing similar issues.
Thanks again