This plant requires moist, but not waterlogged or boggy, acidic soil, and prefers half sun half shade. Not sure planting on a bank will provide the moist soil conditions it requires.

Thanks bamboo....

It is within easy reach of a large rain water butt. If I water regularly from this source would that be OK?

Yes, but to be honest, I'd take the precaution of digging a larger area than you need and adding lots of humus rich material, such as well composted manure or garden compost, etc., to increase the capacity of the ground to hold water and nutrients (Magnolia also likes fairly rich conditions), together with some ericaceous compost to help acidify the ground. And because its late spring already, I'd insert a length of pipe (a piece of plastic guttering downpipe, say, about a foot long) for watering purposes. The way to do this is to prepare the area, sit the plant in the right place, start backfilling the hole a little just to keep the plant in place, but so you're still able to see the rootball. Now take your piece of pipe and lay it at an angle (probably about 25 to 30 deg) with one end very close to the rootball, preferably near the bottom, or halfway down, continue filling in with soil, lightly tread down, ensuring the pipe is sticking out a bit so you can pour water down it. Position it so that its at the back of the plant and can be accessed, but not seen too easily. Water in thoroughly, and when you water from your butt or using ordinary water, pour it steadily down the pipe, repeating this procedure 4 or 5 times, waiting for the water to disperse between each application, as well as watering around the base generally. Doing this ensures that water reaches the roots rather than just evaporating in the top 2 inches of ground when its hot particularly.
Note that the eventual height of this plant is 4 metres or so.