I've just took off my braces however i have a gap less than a mm on my lateral incisor on my upper teeth. will retainers help?

Maybe: Should not be a gap if the braces were just taken off. See the orthodontist ASAP.

Depends....: ....if the retainer impression was taken when the space was closed, wearing the retainer will attempt to close the gap. If the space was there when the impression was taken the retainer will try to take you back to that point. Under certain conditions, space around the laterals is left on purpose. Talk to your orthodontist about it.

Gaps: Not sure where the gap is located.Is it between adjacent teeth,are the teeth balanced in width proportionately with the centrals and cuspids? Is the gap between the biting edges of upper and lower teeth? Retainers can be designed to move teeth to close gaps and then altered to be passive and hold teeth in place. Gaps can be closed with restorative Material, light cured resins or porcelain veneers

Depends: Depends on tooth size (lateral incisors are sometimes undersized), tooth position, bone support, and many other factors. For example if your gums were inflamed during treatment because of hygiene issues, swollen gums can pop teeth apart. Return to your Orthodontic Specialist, the person most familiar with your occlusion, to discuss this.

Get them back in.: Visit your dentist quickly if the trays don't fit. A minor correction can be quick and easy.

Possibly: sometimes retainer can be designed to incorporate the final desriable movements - speak to your orthodontist about this