Hi Dawn,
Unfortunately, Just You are not planning to add a separate beach stay to the Sri Lanka tour at this stage. As you mentioned, Travelsphere's Sri Lanka tour can cater for single travellers, with a single supplement. If this is something you're interested in and have a certain date in mind, I can tell you if there are any single travellers booked or what the supplement will be on that date.
Sorry I can't be of anymore help.
Kindest regards,
Monty