Jonnyjonjon gives good advice !
Make sure the hex key (5mm) is in properly. I thought I had rounded it off at the top because it slipped a little but then I tapped the key with some metal and heavy and got it in properly and the blanking plate came undone once I had a decent amount of leverage with a long hex key.
For my oil temp sender I used 4 copper washers total thickness of about two 10p pieces. The sender was too long for the hole and I didnt want to tighten it onto metal.
I initally installed using steel washers and it worked fine but then swapped over to copper washers which meant taking a load of stuff off again and more scraped hands to get them first and do it right first time !
Powerwise there is a +12v permanent and a switched +12v on the headlight switch wiring block just behind the fuse box. If you want a dimmed voltage feed then there is one on the dimmer switch block just next to it.
Red (thick wire left and top of the block) = +12v perm
White and black (thin right and bottom) = 12v switched. As does the thick red/black next to it. I am not sure which is best to use but I used the white and black one
Brown on the left (top I think) is the earth I used (could have used one from the headlight block) and the blue and white is the switched and dimmed voltage so if you wire up the lighting part of the gauges to that the brightness will change with the dimming switch.
Make sure you dont wire up the sender power to the dimmer switch though as it will mess up the reading.