You need to get the person's CU Sortcode, Account Number and probably their CU Account Number. ie The number with a letter, which you already have.
Now the fun starts, each CU it is own little kingdom with various degrees of banking and technical savvness, from the scale of now being part of the clearing system (Having their own sortcode via BPN Paribas) to 'Mary' dropping into the local branch for a printout of the days lodgments.
You need to setup the Sortcode and Account Number as normal with 365 and depending on the CU (most of them) enter the CU A/C Number is a Reference Narrative. Then with a bit of luck and a payer the money will arrive in the CU Account. Especially the first time, you/they must insure the money has arrived, don’t forget about it. Rant about many failure modes deleted....