Thank you for your responses.
I have had confirmation from the MHRA that it is acceptable to use the CRF as source for certain types of data as long as it is specified in the protocol.
Care must be taken by the investigator to ensure that information important for patient management is transferred from the worksheets/CRFs to the subject case notes, for example, adverse events, however in this case it must be clear which document is the source. Such information is vital for patient management eg when patients are managed outside the trial team by different medical specialities.
If your current protocol does not specify that the CRF will be used as the source (as was the case with our trial) then this should be updated. Since this is normal practice, a substantial amendment would not be required.
Thanks again
Gemma