Inclusion Criteria:  Histological documentation of metastatic (Stage IV) Colorectal cancer (CRC) and  Documented tumour tissue aberration in RNF43 and/or RSPO Documented confirmation of microsatellite stable (MSS) status Patients must have had documented RECIST1.1 defined radiological progression following a minimum of 1 prior SOC treatment regimen for metastatic disease Eastern Cooperative Oncology Group performance status 0 or 1 At least one lesion that is measurable by RECIST 1.1 at baseline Patients must have at least one lesion suitable for biopsy at screening and be willing to provide mandatory tumour biopsy samples Patients with adequate organ functions Female patients of childbearing potential must have a negative pregnancy test prior to start of dosing Female patients of childbearing potential and male patients with female partners of childbearing potential must agree to use a highly effective method of contraception during the study and for at least 5 months after the last dose of study drug.  For patients on RXC004 monotherapy treatment (Arm A) the following inclusion criteria will also apply to enter the RXC004 + nivolumab treatment phase:  Patients must have had documented RECIST1.1 defined radiological progression on RXC004 monotherapy treatment on the first scheduled scan (week 8 +/- 1 week) Patients must receive Cycle 1 Day 1 of combination study treatment within 28 days of the first scheduled scan (week 8 +/- 1 week).  Exclusion Criteria:  Prior therapy with a compound of the same mechanism of action as RXC004 Patients at higher risk of bone fractures Any known uncontrolled inter-current illness or persistent clinically significant toxicity related to prior anti-cancer treatment Patients who have any history of an active (requiring treatment) other malignancy within 2 years of study entry Patients with known or suspected brain metastases Use of anti-neoplastic agents, immunosuppressants and other investigational drugs Patients with a known hypersensitivity to any RXC004 excipients Patients with a contra-indication for denosumab treatment Patients who are pregnant or breast-feeding Use of any live vaccines against infectious diseases (e.g., influenza, varicella) within 4 weeks (28 days) of initiation of study treatment Patients with a mean resting corrected QTcF >470 ms, obtained from triplicate electrocardiograms performed at screening  For patients on RXC004 + nivolumab combination treatment (Arm B or Arm A RXC004 + nivolumab treatment phase):  Patients with any contraindication to the use of nivolumab Patients with active or prior documented autoimmune or inflammatory disorders within the past 5 years Patients with active infections, including tuberculosis, hepatitis B, hepatitis C or human immunodeficiency virus Patients with a history of allogeneic organ transplant or active primary immunodeficiency Patients with a known hypersensitivity to nivolumab or any of the excipients of the product