Problem re-installing High Sierra

I was given an old (circa 2012 I think) Macbook Pro with High Sierrq to tide me over while my desktop was being repaired. It works without problem except that it is extremely slow because, I think, the previous owner had loaded it up with all sorts of apps, many of which didn’t seem to have any use. I decided to do a re-install of High Sierrq to see if that would result in something a bit faster.

I followed all the Apple instructions to do a re-install from the Recovery console. The first attempt to re-install High Sierra didn’t work and the Apple instructions said that, if this happened, delete everything on the Mackintosh HD disc. So far, so good - this brought up a window with High Sierra logo and instructions to click on “Continue”. However, this resulted in the message that the Recovery Server Could Not be Connected”. Changing the format of the disc to APFS encrypted did not help. have tried everything on various internet forums and it seems that the solution is to download a High Sierra installer using a machine that is compatible with High Sierra. Great if you’ve got one but my desktop is an M3 iMac and well beyond that. 

There is apparently a long and very tortuous method that involves terminal commands to allow the Recovery Server to be connected but I am reluctant to try this in case it caused more damage. All the links to High Sierra installers in Apple pages seem to be broken. 

This seems to have been a problem that many others have had so I am hoping that a solution has now emerged. Can anyone suggest an easy fix (yes, I didn’t do a Time Machine backup, slaps wrist)?