The seed bank can lie locked away in the soil for decades, but as you've highlighted, other species can become predominant in the intervening years since the original woodland was felled and replanted. Brambles can be part of the existing seed bank, and can get out of control, but they play a vital part in the regeneration of any woodland.
Sadly, if the whole wood is under a a Tree Preservation Order there's not much you can do regards felling. Seems odd that there's a preservation order on a conifer plantation though, unless its in a SSSI or AONB.
I suggest you take a look at Chris Starr's book on woodland management if you don't already have it;
http://www.amazon.co.uk/gp/aw/d/1861267 ... -1&pi=SL75This should help you to understand management of the site.