Lawn Tractor - daft question

Drastic times call for drastic measures. If trying the lever doesn't work, following the mechanics, and asking on pistonheads and ringing the guy who owns it all comes to nothing, then you have to throw in the towel, and look in the book.
Lots of little drivy things tend to have a manual tucked away on them, under the bonnet, sealed in a plastic bag along with other unneeded paper work, like warranty and service records.

Now that its cut just go over it with your new mower with the cutting deck set quite high. You may need to go over it a few times, lowering the deck each time, until you get the finish you require. I would have thought the same procedure would have worked with the original long grass.
What width deck did you get? I have an Iseki TXG23 with a 60" and only have 1.5 acres to mow.