Double Taxation applications and claims: Self Assessment: Repayment claims from non-residents: What to do if you receive another claim while an enquiry is open
If you receive a further claim while an enquiry into a previous claim is still open you should be aware that you might need to open a separate enquiry into the new claim. It may be that a full response to the enquiry already open will provide all the information you need to reach a decision about the new claim. However that will only be relevant if the information enables you to accept that the previous claim was correct. If not you would need to open an enquiry before you could amend the new claim to nil or disallow it.
To avoid possible complications later it is probably best to open an enquiry into the new claim and say that you will not be able to proceed with the new claim until you receive the information requested in connection with the previous claim. For the avoidance of doubt you could enclose a copy of the enquiry letter into the previous claim or re-state the information that you require.