Re: Finding the first customer......
Tend to agree with Lorz.
Getting relevant experience and a knowledge of the area (which can be displayed to the customer) would be of huge benefit to picking up clients.
If you have experience in the area finding a way to convey this to potential clients is needed, if not, I'd suggest you try and build some up before spending too much time on marketing.
By gaining experience with the likes of WYG or any of the Environmental Consultants in the country (there are a huge number out there) you'll not only gain experience, you'll also add a number of potential clients to your list who will have first hand knowledge of your work and therefore be far more likely to employ you.
Other little things like stating your own/your staffs chartered CIWEM (or any of the other institutional bodies) status may be of some (slight) benefit in gaining confidence in potential clients.
The forestry side of things may give you a specialism that not many of the large groups already possess. The chances of sub contracting here seem quite good so postioning yourself in a good working condition with large consultants (RPS, Jacobs, WYG, etc etc) could provide a good starting point for ICM.
Best of luck, hope it all goes well.