Ct scan shows diverticulitis gi advised sigmoidoscopy to check narrowing of colon could it be cancer as been told only scoping can confirm difference?

Unfortunately those: two conditions can look similar on the CT scan. Another option is to treat for diverticulitis, then repeat the CT scan to see if the abnormality has resolved. If you don't have symptoms of diverticulitis, you should have the sigmoidoscopy.

Yes: Given your young age, diverticulosis(itis) is not very common, but can occur. Is there any colon cancer or polyposis in the family? If CT shows an abnormality then I would strongly recommend a colonoscopy to investigate further, especially if there is narrowing of the lumen (inside diameter). The test is relatively low risk and can provide a lot of information and peace of mind.

Scope is a good idea: Although the imaging technologies such as MRI and CT Scan can give us a lot of good information and are extremely useful they do not allow for a biopsy which sigmoidoscopy does. A biopsy can give a lot of information. I recommend following the advice of your gastoenterologist. I wish you the best of luck!