
Hey Graham,
Welcome to PCHF. We have a wonderful team of techs here, and I feel certain that we will be able to help you determine why you are having this trouble.
Here is the formula as I entered it into Excel
=TEXT($B$2,"mmm")&CHAR(13)&TEXT($B$2,"yy"), I also tried it with only the B absolute reference you have, and no absolute reference at all. All three ways I got the result you are looking for.
Try this, first, make sure that you have not overlooked the comma between the B2, and the "yy". Then make sure that you don't have any extraneous commas. If neither of those are true, try adding the letters SET to the end of CHAR, this will bring up a function error code, during the process of resetting the function, Excel may reanalyze what it is supposed to be returning.
Looking forward to your reply,
TTFN
LGW