The relevant regulations are The Road User Charging Schemes (Penalty Charges, Adjudication and Enforcement) (England) Regulations 2013
These Regulations make provision for the civil enforcement of a penalty charge imposed in respect of a motor vehicle by a road user charging scheme made under Part 3 of the Transport Act 2000 (c.38).
and also
The A282 Trunk Road (Dartford-Thurrock Crossing Charging Scheme) Order 2013
Sections 163(3)(d), 167(1)(a) and (2)(a) of the Transport Act 2000 authorise the Secretary of State to make a trunk road charging scheme in respect of roads for which he is the traffic authority an...
There have been two minor amendments since but nothing to change the basis of the Order
Two points to be made; (1) there are no powers in the Road User Charging regulations for an unpaid toll fee to be requested on a PCN. (2) a PCN can be issued when the toll fee is paid late, but this cannot be held against a future crossing and the toll fee be requested again. The PCN is for paying late, but the toll fee has been paid. This was decided at the Traffic Penalty Tribunal by the Chief Adjudicator Caroliine Shepherd, no less, in 2018, but there has been a subsequent adverse decision by what I rank as a teenage scribbler adjudicator who seemed woefully ignorant of the regulations.
However, having said the above, the A282 Order has this to say on PCNs: -
Penalty charge for non-payment of charge under the second charging regime
8.—(1) A penalty charge of the amount shown in Part 1 of Schedule 4 is payable in respect of a motor vehicle upon which a charge under the second charging regime has been imposed and where such charge has not been paid in full before midnight on the day immediately following the day on which that charge was imposed.
(2) A penalty charge payable under paragraph (1) is—
(a)payable in addition to the charge imposed by the second charging regime;
(b)to be paid in full within the period of 28 days beginning with the date on which a penalty charge notice relating to the charge that has not been paid in full is served;
(c)reduced by one half provided it is paid in full prior to the end of the fourteenth day of the period referred to in paragraph (2);
(d)increased by one half if not paid in full before a charge certificate to which it relates is served by the Secretary of State (as the charging authority) in accordance with regulation 17 of the Enforcement Regulations.
The charge imposed by the second charging regime is the toll fee. The second regime came into effect when the toll became payable on-line rather than at a toll booth.
Note the title refers to non-payment, not late payment. My opinion is that requesting a second payment of the toll fee is wrong in law as payment has been made albeint late, and the PCN is a penalty for late payment, not non-payment.
Thats my opinion anyway, but wait and see what the others say.