This 2011 13" MBP was described by the seller in their eBay listing as non-working and only suitable for stripping down for parts because it no longer booted, as can be seen here with the flashing question mark and folder icon.
Credit where credit is due, members of the LEM FB group confirmed my suspicions that the fault lay with the SATA cable rather than anything serious and so I took the plunge and gambled with £90 GBP. After all, even if the suspicions turned out to be unfounded, I could always re-sell it to someone who needs a donor machine because as you can see, the condition is immaculate.
After replacing the SATA cable and installing a new drive, I cloned my existing Snow Leopard - El Capitan dual boot setup from my MacBook 5,2 with Carbon Copy Cloner via FireWire and Target Disk Mode, rebooted the MBP and waited for the moment of truth.
Hurrah!
Everything works perfectly and the finishing touch was booting into Snow Leopard to flash the Superdrive to RPC-1 so that I can watch whichever DVD I please without the annoyance of region restrictions. (On particular drives, VLC can't always bypass this issue unaided, so flashing to RPC-1 wherever possible works best.)
If I want to purchase a legal copy of a film from wherever in the world and watch it, no-one is going to stop me. Especially when it involves classic films.
I'm using this as my new driver for academic work but I'll keep the trusty MB 5,2 as a backup. The addition of Thunderbolt and FW800 are nice bonuses to say the least.