Looks like I've got it. Wouldn't turn this morning so I tried the fuse 18 to no avail. After a few goes it eventually turned and started, sweet as a nut.

I read all the fixes on the net and they all appear temporary although the Powerful UK fix with liberal amounts of WD40 on the mechanism maybe a longer term solution. Our own Cam Tec Craig is acknowledged on the Powerful UK video so perhaps he can throw some light on the longevity of the "fix.

First instruction from my indy was to get the battery charged to peak so it's currently sitting on the C-Tek although it started fine once the key turned without any labouring from the battery.

Let's see what happens when it's 100% charged. 2017 5.0 Autobiography Supercharged
2005 Merc SL350
2005 Disco HSE 2.7 oil burner

When it happened to me, on a TD6 granted, it was a new ignition switch. John
2008 Stornoway Grey 3.6 Tdv8 Vogue
2005 TD6 Java Black Vogue - Written off!!
GAP iiD BT
2003 Discovery TD5 Auto, Nanocom Evolution - gone to a new home!
MasseyFerguson 152 - No electronics!! - Sold

Not the best idea to put wd40 in any keyhole, just attracts dirt. Get some graphite powder in there, grind down a pencil lead if none available. 2007 TDV8 VSE
2003 TD6 gone.
2002 P38a gone
1999 P38a gone
1997 p38a gone
1993 VSE gone
1992 VSE gone
1966 Series 2a with V8 conversion gone

Not sure which model you have but the following link shows I solved the same problem with my 2006 TD6 L322:
https://www.fullfatrr.com/forum/topic49707.html

After keeping the C-Tek connected for a few days the key has turned faultlessly so I've replaced the battery this morning with a Yuasa jobby from Halfrauds. Only £105 with their trade card and a 5 year warranty. The old battery went in in 2011 so it was on borrowed time.

Bit of a job to fit due to size and weight of the dammed thing. It must weight 25 kg.

All up and running and starting as it should. Let's hope it stays that way. 2017 5.0 Autobiography Supercharged
2005 Merc SL350
2005 Disco HSE 2.7 oil burner