Roots been left in after tooth extraction. can this lead to infection, i have a red line goin across my gum and painful sornes on side of palate?

It is possible: It is possible to get an infection however if the root tips are too close to a nerve or the sinus it is sometimes better to leave them in rather than to risk some possible permanent nerve damage or a sinus perforation. I would return to the dentist that did the extraction and have the area rechecked.

Retained roots: Retained root from teeth that were extracted can lead to post op infection or problems down the road. See another dental surgeon for a second opinion.