Thanks to everyone who replied.
I should say, It's my own land, so I can plant trees without worrying if I'm breaking any rules.
I do have a strimmer, but it's electric and I'd need a 500' extension to reach to the nearest socket. I guess I'll either have to get a petrol strimmer, or cut the brush down manually. Not sure what type of tool would be appropriate for this?
After reading all your replies and thinking about it, I'm thinking that a mechanical digger would be overkill.
So I could (as allotmentann suggested) clear the scrub, cover it over with weed control fabric, and dig holes for the trees and have them planted in the ground this autumn?
I'll do as Trillium suggested, and compost what I cut down. Providing I don't find any complications (I'm not sure what's the worst I could find under the brush?) I'll go ahead with the rest of the plan.
I'd like to have the space around the trees covered in lawn eventually, so I guess once the fabric (and mulch underneath the fabric perhaps?) has done it's job, get a rotorvator and use it to smooth out the top layer of soil so that the lawn can go on top.
Does that sound like a good plan?
rawrecruit - Thanks for the links (going to have a read of them after I post this), and for the heads up regarding watering. I hadn't considered that aspect. I'll probably have the same problem with the veg plot next to the orchard!
I'm hoping that soon after the trees are established their roots should have found the water table which is quite high on our land. The trees are a bit lower (perhaps 1-1 1/2') below the veg plot, so should be closer to the water table.
We already have two (approx 60l) water butts (and I'm thinking about putting in more, as well as storing water in bins when the butts fill up) which are uphill from the plot/orchard, so I'm thinking I could just run a 500' hose down to them.
I was also considering building a small reservoir which could either be fed from guttering of the house and/or from land drainage pipes which I'm considering installing on a boggy patch of our land.
The other option is getting a license to abstract water from the small river that runs along the boundary of our land, and is perhaps 40-60' away from one end of the planned orchard.
I think options 1 and 2 make the best sense, as during a very hot summer there is a danger of the river drying up, and I'm pretty sure I could only abstract water during the wetter parts of the year anyway.
Thanks again for all the replies and suggestions!