memory basic 8 gb

I hope you purchased very high quality RAM, iMacs are extremely sensitive to using quality RAM. Most experienced users on these forums only use and recommend either Crucial.com or Macsales.com RAM. Why, simple...reasonable prices, terrific service, lifetime warranty and above all they work.

Actually Apple should not've been selling 8GB Macs of late, terribly awkward in this day of bloat.

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Whew, they've changed pastebin & made it harder, but after pasting in, click Create new paste button, then Embed button, then copy the URL...

<script src="https://pastebin.com/embed_js/KuvnghqA"></script>

The important part of the above would be...

https://pastebin.com/embed_js/KuvnghqA

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...

Please read the following user tip and restate your question/problem in detail:

Writing an Effective Apple Support Communities Question - Apple Community  

Then you should use OWC RAM, they have international distribution of including the UK. Here is the link OWC Europe