I am referred to an ENT due to long term swollen lymph node. Ct done, bloods normal. I have now found another one on other side of neck? Worry?

Biopsy? No need to worry yet. Just return and see if your ENT thinks a biopsy is indicated to rule out Hodgkin's lymphoma which can happen after mononucleosis sometimes. Fortunately, that is usually treatable, even curable. Please do make sure your hiv test is done and negative, though.