Since I had been looking at another way of doing this, I might as well post my solution. This approach is designed to work real-time, i.e. it will automatically update your resulting text whenever the value in X3 changes.
I have included a spreadsheet that demonstrates it. To see it work, change the value in H3. To see the code, go into Tools , VBA.. There are 2 bits of code, the MakeBold subroutine and a bit of code in the ThisWorkBook section.
This is what happens. It does the concatentation in cell A2. The value in A2 is then copied and pasted into A4. Then it looks for the text following the last "L" in cell A4 and makes it bold.
If you have trouble running th procedures, you may have to set Macro Security to low.
Cheers
Rob