Update on my question.
When my hard drive started to fail I started it in internet recovery, erased and formatted the drive, and then attempted to download High Sierra. This had worked in the past when the drive started to go wrong about a year ago and the Apple shop said it could not be fixed. Well I did manage to fix it for a year. This time when I formatted the drive I could not load on High Sierra and that was the reason why I started this question. What I could not get my head around was that if I had formatted the drive then that meant that the internet recovery option could not have been on the drive because if it had it would have been erased, so it must have been embedded in the actual computer itself. The Apple website says that the mid 2010 27 inch does not have the capability to start in internet recovery mode, so why was I able to do it?
I have now replaced the faulty 1TB drive with a 500GB SSD. Powered up holding Command/R buttons and the spinning globe came up. Then the internet recovery screen, then disk utility. Checked the drive and the basic Mac OS was on there so I formatted the new drive, clicked install High Sierra and thats exactly what it did.
Yes this is an old MAC Mid 2010 but I have managed to repair it for £108 (about $150), which is a fair saving on a new computer. We only use it to browse the internet and send the odd email so even for an old unit it is way over spec for what we actually need, but best of all it has the big screen which is great for old guys like me.
Thanks for all the replies and good luck.