First of all, at 40 you are still young! You still have 25+ years left in the work field if you decide to work at least until 65. Second, employers don't care about the age but about your ability to perform the job and deliver.

Here are some off the cuff thoughts on your situation:

• If you like what you're doing (the role of the business systems analyst) then I would definitely recommend that you get some formal training in business analysis.
• Since you already have 5+ years in a BA role, you should consider a training course which aligns with the BABOK and which would also prepare you for the CBAP (Certified Business Analysis Professional) certification offered by the IIBA. = > the training and the certification will help you fill the gaps in your knowledge and will prove useful whether you choose to stay with you current employer or move somewhere else. 
• Business analysts with vertical domain/industry knowledge are much more valuable than those who don't. Since you've been in the publishing industry for over 17 years I'll have to assume that you have a decent understanding and grasp of the business. That is a strength (given that you like the publishing industry). 
• => you could position yourself as an experienced Business Analyst with strong knowledge of the publishing industry and with over 17 years of experience in this field.

My gut feeling tells me that you should stay with your current employer until your take one or more business analysis training courses and you feel comfortable with the new methods, constructs, and tools you learned about. Since you already have the job and title of BA, use the current employer to practice what you are learning. This could go a a few ways: your current employer might really appreciate your new skills and your ability to help them formalize the role of the BA and related processes and procedures (more money, bigger role, etc) or this would be your stepping stone to move for a different company (within our outside the publishing industry).

Hope this helps!
- Adrian

You are very welcome!

I believe that, in the long run, the IIBA certification will have more recognition than the ISEB. Having said that, they are very different and you can have both.  In order to attain the CBAP one has to have the work experience and take a test.  I don't believe the ISEB requires work experience just that you take the courses and you pass the test.  So the ISEB would be suitable for somebody with less work experience in business analysis.

For the time being, I would say that going through the ISEB program would be a good way to go for you since you not only gain the ISEB diploma but it also forces you go through the 3 course courses: Business Analysis Essentials , Requirements Engineering, and Organisational Context.

Once you complete get your ISEB diploma you can always apply for the CBAP certification also and take the test.  That way you'll have both!

- Adrian