Thanks for you advice Jack. However, you are not totally correct, although the cost may be so prohibitive the effect may be the same. I read on the Apple.com's Advice about Memory and RAM for the for Apple iMacs that it specifically states"the "Early 2013" and "Late 2013" 21.5 inch iMac models "has no user accessible RAM compartment " and so " RAM replacement on the iMac 21.5 inch ( Late 2012, Early 2013v and late 2013, Mid - 2017 or 2019 )MUST be done by an Apple Retail Store or Apple Authorised Service Provider" . That is the reason that I enquired as to whether anyone knew what such a procedure costs. The maximum increase allowed is 16GB instead of the current 8GB . I have a Late 2013 21,2 inch iMac and so I am most interested to find out . You are partly right because the same Apple.com site says that "It is not possible to upgrade thew RAM in the "Mid -2014 or Late 2015 21,5 inch models at all after purchase.
Mad Memory