V6 will not crank after flat battery

[Mrs Feynmans Bongos]

Yesterday my V6 would not start or even crank, including with a jump applied.
I found interior light switch left at ON (ie not "Door"), measured the Battery at 5V
(It need only a tyre and front suspension drop links on MOT mid-Aug, then 800 faultless miles to the Lakes, before one of us left it on

But today (with the flattie swapped-out for charging) it's the same using a spare Battery @12.5V.
**Nothing** works apart from Headlights (battery drops to 11.9V), but it stays @12.5V when Key is turned to Start (does not drop a millivolt).
Auto Selector is in "Park" (where I left it), waggled it but could not move it.
All FUSES under the bonnet bleep when tested ie are short/good. Cabin fuses next ...

Advice appreciated.
Eg could some Relay have stuck? Surely a failed Starter Motor would still drop volts?

[g8dhe]

Is there a clicking as you turn the ignition switch to start? If so then suspect the starter motor contactor's contacts, spares are available from eBay ( provided your not a Northern Japan vehicle with heavy duty starter).

[teenmal]

Personally I would make sure that the engine is in fact turning (manually) and check earth and battery connections and battery posts(terminals) for damage and as Geoff said the starter solenoid .