A year after 'war and peace' (above) and all is well. SSD works a dream. Have been using 'Fan Control' from day 1 with no issues to date.
So to the moment of truth. Booting up in recovery mode.
I heard the start up chime but the screen remained black. Back on the Internet to check diagnostics (noticing comments at the top of this thread). On further inspection after taking my Mac apart a 2nd time (in a fraction of time it took originally) I noticed that my nemesis (the display data cable) had either come out on reassembly, disassembly or had not been plugged in correctly in the first place. Several slow mo web videos later I finally nailed it.
I'm currently running a 'Time Machine' backup (6 hours with the fan running at full tilt) but am hopefully on track.
Will report back if there are issues. No news will be good news.
Complications arose when connecting back up (the part that isn't in any guides).
Once again, the LCD sensor cable was tricky fuelling concern of possible damage.
The display data cable cost me a good chunk of afternoon. Without the iMac being upright on it's stand (OWC method of process) it's so difficult to see where this wafer thin cable should connect. I tried moving various props to hold the LCD at a different height and angle as well as moving an LED spotlight about (red hot). The Airport card also obstructs any clear view of both plug and socket. On top of that there's the locking tab to get back in place without touching the logic board.
Some time later, I was fairly (but not 100%) sure that it was locked in place.
After plugging in the other cables, keeping the screen and glass dust free was also quite challenging.
Display and HD temp sensor came out without any bother.
My dead HD also came out without too much trouble although the SATA and thermal sensor cables were quite stiff.
I should mention (at this point) that I had decided to replace my old HHD with an SSD.
I therefore decided to completely remove the thermal sensor cable with a view to using 'fan control' software after replacement. At first, I assumed that the drive would be facing upwards and consequently put the sponge from my old HD on the outside of the 2.5 - 3.5 mounting bracket I'd bought. This also lead to me putting the upper mounting bracket on the wrong side when in actual fact, the mounting bracket would fix back in with the new SSD hanging upside down and sponge on the drive itself (not that this matters. It will ultimately be on it's side when fitted anyway).
With new bracket in place, the SATA cables were a little bit short of where the plugs were. The plugs needed forcing in to line with the sockets. Some slight manipulation required here.
After peeking under the LCD I was alarmed to see how short the vertical sync cable was in mine (4-5 cm from top to bottom). It turned out to be quite a long cable but I had to prize some of it away from some adhesive/adhesive tape towards the bottom.
The LED backlight power cable was very stiff to remove. I had to be careful not to force it even when pressing quite hard to unlock (might have been me?).
Next up, the dreaded display data cable. No problems disconnecting this although it would later give me a big headache.
I found the LCD sensor very fiddly. I have pretty small hands yet, fingers/finger nails seem inadequate for such a small cable and socket. Of all the cables I removed this was the one I expected to have damaged. I pulled on the cable through frustration as I couldn't get a hold of the plastic plug.
I'll always choose the DIY option as I don't like paying for labour. This time (with virtually zero experience of taking anything apart, never mind my precious iMac) I wondered if I'd bitten off more than I could chew. I spent a good bit of time watching videos and reading walk through guides before attempting a HDD - SSD replacement. This guide was one of the best and I feel I owe an account of my experience to anyone thinking of doing this.
Removing the glass was as easy as I expected it to be. I used 'the straw' method (mentioned in the comments here) to remove the screws, avoiding magnet attacks. When in the straw, you can quickly whip out the screws using the magnetic tip of your screwdriver. 2-3 were claimed when screws or straws slipped although it was easy to get them back with tweezers.