Yes, if you want your work to comply with 17th edition then your modified circuit should have RCD protection.Page 55 of the On Site Guide lists the requirement for RCD's;
"RCD's are required;
i) ...where the earth fault loop impedence is too high to provide the required disconnection time, eg, where the distributor does not provide an earth - TT systems,
ii) ...for socket outlet circuits in domestic and similar installations,
iii) ...for circuits of locations containing a bath or shower,
iv) ...for circuits supplying mobile equipment not exceeding 32A for use outdoors,
v) ... for cables without earthed metallic covering installed in walls or partitions at a depth of less than 50mm and not protected by earthed steel conduit or similar,
vi) ... for cables without earthed metallic covering installed in walls or partitions with metal parts (excluding screws or nails) and not protected by earthed steel conduit or the like.
30 ma RCD's are required for ii to vi above"