In the case of Dartford yes there was!
As I said the original acts of Parliament authorising the construction of the bridges and tunnels were very specific - it would have been illegal to continue to charge motorists once the capital construction costs were paid.
NEW legislation was thus essential if the Government were to keep their nice little revenue generator and hence the Government ensured a fresh bill was laid before Parliament to do just that.
More widely the ability to continue to charge motorists for ongoing maintenance has to be factored into the authorising acts. If said acts include a stipulation that the tolls are to pay for capital construction costs then any half competent lawyer will quickly be able to prove the charging authority were acting illegally.
As to the question of whether motorists should pay for ongoing costs - in the UK have a long established system where those costs are funded by Central Government. Why should people have to pay to cross at Dartford and not at the Thelwall viaduct (M6), the Devils Punch Bowl Tunnel (A3) to name but two structures which are going to have higher maintenance costs than most of the rest of the strategic road network.
Its a different situation in places like France where road tolls are seen as the norm, even for pretty boring / cheap to build motorways as its clear the system as a whole is geared up to recouping such costs directly from motorists on a massive scale.
While I personally prefer the French model, I am also very much aware that ship has sailed over 50 years ago so to speak and as such attempts to impose road tolling retrospectively in the UK (outside of to fund the construction costs of major bridges / tunnels or true congestion charges to encourage public transport use in large cities) is not acceptable.