Andy,

It is difficult but no impossible to make a tri-gear trainer to take-off from less than perfectly trimmed grass with an OS LA 40.

If you need to keep your LA 40, you could do as I did:

I was doing it for a while, but just after I replaced the stock wheels by lite (foam) type 3" diameter for the main and 2-1/2" diameter for the front wheel.

I also had to change to a 11x5 APC propeller, and hand to extend the gear down so the prop's tips clear the grass.

The location of the main wheels respect to the CG (assuming the CG is properly located per manufacturer's instruction) should be close enough as to facilitate the rotation of the plane when you input elevator up during the last phase of take-off.
It is also very important to beef-up the gear, so it does not bend towards the tail during take-off.
With the fuel tank empty, the plane should turn nose up with a light push on the tail, and then return to normal position by itself after release.

In normal take-off position, the wing must have a small but positive incidence angle respect to the ground.
If not, sliding the front gear up or down, can achieve that attitude for the wing and fuselage.

As stated above, any experienced pilot of your club know how to check those little but important details.

Read this article:

http://masportaviator.com/2006/08/09...d-up-part-two/