That's a huge question you've posed mate....
Firstly all sinking lines will keep sinking if you leave them.....it's just the actual sink rate that varies (ie 3 " per second for a Di3, 5" per second for a Di5 etc)
When using a sinker you should aim to get the majority of your takes in the middle half of the retrieve...
This will mean that you are fishing at the depth the fish are holding at.
Also you will pick up some fish at the start of your retrieve that are higher in the water or come up to take the fly.
And at the end of your retrieve - the fish that are holding slightly deeper in the water.
It's worth remembering (if your flies are unweighted or boobies) that your top dropper as it's closest the fly line will fish the deepest; as it follows the line down.
If you repeatedly catch only on the top dropper let your line sink an extra ten seconds before retrieving...this way you should get all your flies at the right depth.
But sometimes they all take the fly as it changes direction - ie on the sweep up)
line markers are essential for the hang at the end of the retrieve, it allows you total control of your flies and depth..I have mine at 13ft (the easy way is to Tip-Ex a 4" section of your line and use it as a line marker....this will just scratch off with your nail if you put it in the wrong place)
Keep an eye on your sinking line as it enters the water, the angle will give you an indication of how deep your fishing
The trouts angle of vision will allow it to see up as you thought...but in this hot weather the fish may not want to come up in the warmer water..with less oxygen..so it's better to present your fly at their depth..
Although on the midlands reservoirs the fish are still quite high considering the temperature etc..
You could use a floating line and a weighted fly to get deeper...however everytime you retrieve the fly will rise in the water (especially so if you pull hard) so taking it away from the fish and out of the zone...so you need to compensate and fish a slightly longer leader (ie a 14ft leader with a goldhead when retrieved will present your fly anywhere from 5-13ft depending on the speed of retrieve and weight of the fly)
Just try to visualise your fly, and flyline and work out the depth their fishing at...and try to get it where you imagine the fish will be..
For example whatever line you choose, search the depths....let everything sink for 5 seconds......then 10 seconds....then 15 seconds etc...be methodical until you get it right.
Or ask someone esle who's catching