The mark belongs to the partnership of John Langlands I & John Roberston I of Newcastle. They were in business together from 1778 until 1793 when John Langlands died. The business continued for another two years with his widow, Margaret replacing her husband in the running of the business and I believe still using the IL over IR sponsor mark.
They registered numerous marks in both London (registered March 1780) and Newcastle (registration date unknown)both with and without pellets. The IL over IR with pellets mark appears to have only been registered in Newcastle placing your spoon as probably Newcastle assayed.
Further, the lion passant looks to be the type used in Newcastle rather than London during the last quarter of the 18th Century. I think it changed around 1790 (someone please correct me if I'm wrong with this) , placing your spoon somewhere between 1790 and the earlier 1786 when this Geo III duty mark was introduced.
Much of the above information was found on Phil's excellent website; http://www.silvermakersmarks.co.uk
plus small details from others.