Hi Moira
I believe Sulcatas should be provided with temperatures between 23 and 29C during the day with access to a cooler area. At night the temp should not go below 16C. I am taking these numbers directly from the Sultaca Station caresheet here:
http://www.sulcata-station.org/pdf/sscaresheetrev.pdf
I believe this to be good info but as I say, I don't keep Sulcatas myself. There will be people on here that keep them but they haven't noticed you yet. I hope you might be able to get some chat going with people more familiar with these lovely tortoises. Feel free to check out people's profiles to see what species they keep, or search the forums using 'sulcata' and 'sully' and see what and who turns up!
But, yes it looks like you will need additional heat for your sully. Ceramic heat emitters work well for night time for many of us and they can also be used during the day. I don't know if you have a warmer room that you could move the enclosure into? Even just for winter? You can also provide additional heat with halogen lamps which I am a fan of. These don't have uvb of course but they can be great for heat.
Hope this helps a bit!
Edit: Just seen Danny's post and his advice will be better than that which I just pinched from the Sully Station caresheet.