So the access lane to your house has been in use since the 1960s & crosses some forestry land? I find it extraordinary if nothing has been done before to formalise this. Your solicitor says you have proof of right of way. What does this consist of, & is it registered at the Land Registry? If it is not, then something needs to be done.
Look at this thread:
http://www.theanswerb.../Question1184056.html
The circumstances are different but the principle may apply, & it might be quicker/cheaper etc. than negotiating an easement document with the owner of the land.
It is really a matter for your solicitor to sort out with the buyer's. So long as your right is registered I cannot see the need for anything else.