I forgot to mention that I followed the advice of one of your previous commenters and detached the LCD cable from the LCD end not the other end as in your guide. It was perfectly easy but I had to replace the sticky protector with ordinary sticky plastic tape.
It's surprising the hard drive in my mid-2007 iMac lasted as long as eight years. But it became so slow that even typing often produced the beach ball of death. Memory upgrade to 4Gb had minimal effect. So I nearly spent £1600 on a replacement up to date iMac. Then I spotted the 9to5 guide http://9to5mac.com/2015/02/13/how-to-swa... and subsequently yours.
I have no experience of the insides of computers but could find no local expert to do this work. Trying for help from the nearest Apple dealer was a no-no. They just want you to buy their latest machine.
But, with the help of your guide (and my wife), I did it myself with no problem. The tools cost about £20 and the SSD £250 (a 1Tb Samsung 850 EVO), plus a small amount for the TRIM enabler.
I now have my fantastic iMac back again and don't even have to listen to the whir of a hard drive - or a fan!
Thank you so much for publishing this information.