Looking at buying a crankset, and the 165mm is in stock and 170mm isn’t, and there is a bit of a wait for it. I’m tempted to go for the 165 as I’m impatient…so question is; would I notice the 5mm difference out there on the trails?? Can anyone shed some light on it for me? I’m confused!

that’s bloody tiny! If you’re normal height then 175mm is normal..
I’ve got bikes with 170, 172.5 and 175 cranks, and you can feel the difference.

When do you feel the difference bike whisperer? I’m guessing it just makes pedalling harder?

The shortest ones do a bit constrictive on thrutchy climbs, but are fine if you keep the cadence up.. The two bikes with 172s just feel right, and the 175s are on the singlespeed, where the extra leverage helps. You can do a calculation (which I can’t remember..) to turn your true inside leg measurement to your ideal crank length.

We run 165mm on DH bikes as low BB and lots of travel, also not so much pedalling. Most people / bikes have 175, I run 180 on the single speed, Jane also runs 165s but inside leg 26-27″.
So answer depends on your leg length, sort of pedalling and riding.