Don't really understand why you want confirmation of this Hopkirk. (Used to love the programme by the way!) It is against the law to tow anything and drive in the 3rd lane. BUT, of course he would be able to use the 2nd lane to overtake, for example on a dual carrigeway.
Even if there is a hold up and a lorry/coach/caravan in the 3rd lane can still be prosecuted - even if the vehicle was staionary at the time! They must NOT use the 3rd lane unless directed by highway signs or by a police officier.