Ok for those who didn't know, the switch ended up not being the issue

so after many weeks, more oil changes/flushes, new filters, sump off/strainer removed/cleaned etc etc with no improvement (actually it got worse) I finally had the pump taken off and:
[ame=http://www.youtube.com/watch?v=nh5DnOjpNpY&feature=youtube_gdata_player]Range Rover P38 4.0 V8 Oil Pump Broken - YouTube[/ame]
So after around 200 miles or so and numerous starts from cold/hours of standing, I am feeling that she is fixed and the issue is now resolved

currently running 10W/40 semi which the indie put in and the light goes out within a second or so when stone cold and instantly when warmer, so pretty chuffed.
I had the indie check a couple of big end shells at the same time and although some wear was visible (slight discolouration across the bearings - no copper) they are still all good and the rattle at start up has gone, however, looking at the gear in the video, it would make sense that it was the pump all along.
Many many thanks again to all that helped me and gave advice (ESP Rover and Wammers via PM) I am just chuffed she is finally fixed and hope she stays that way.
Hope my issue/resolution also helps others out in the future, especially as I had no luck with finding anyone else with the same issue over the net (similar but not the same) lol