Traffic Signs Manual Chapter 3 explains the requirements for RPZs clearly, but the actual law is in TSRGD:
RPZ entrance signs are covered by S5-3-2 on page 98 of TSRGD. This requires them only to be used for a “Restricted Parking Zone”, a term which is defined in Schedule 1 on page 30. Clause (b) of this definition requires that “none of the road markings at items 1 to 4 of the signs table in Part 4 of Schedule 7 has been placed.” These S7-4 markings it refers to are single and double yellow lines and ‘kerb blips’. Hence, it’s not legally an RPZ if you do have yellow lines in it and therefore it was not legal to place the RPZ entrance signs for it.