no problem use the voltmeter and get some one to press the button (take the keys out of the ignition...oh hang on you have been doing this i take it) and check for 12v when the button is pressed.
Yeah there should be a light.
To get to the solenoid you can pull the carpet back and reach the connector then
Never assume anything put the meter to v and see if the current is on both sides.
Pretty sure there should be a light but tbh i have never looked to see if mine is lit in 5 years lol
on the fuses when removed you need to check continuity ie when the prongs of the tester are joined it beeps, the same should happen on a fuse if it doesn't it's blown
Try each setting all my meters have beeped even a crappy £2 one i had
The filler opening fuse is fuse D, which is the red one at the top of the relay board in Lee's picture. Should be a 10 Amp fuse. Can't see from here but it may well be a "thermal resetting" fuse, a kind of circuit breaker that is supposed to reset itself. There have been reports of them failing from others on the forum.