I have been issued a German family reunion visa type D. it clearly states that the visa is for Germany and not Schengen states as I used to get in the past. Can I travel by bus from London through France to Germany with this visa?
Yes. While I can't find anything useful on a German site, family reunion (type D) visas are issued by all Schengen states, and the French foreign ministry site says:
The long-stay visa is not a Schengen visa ; it is a national visa that entitles you, whatever the reason for your stay, to live in France for more than three months.
The visa also entitles you to transit through another country in the Schengen Area on your way to France and to move freely throughout the Schengen Area for its entire period of validity.
The main reasons for issuing this type of visa are study, work and family reunion.