Thanks for the replies and interest in this situation. More test results follow:
The link (not broken across lines) as received in SquirrelMail: (the domain in the link is a working test site)
http://www.my-test-domain.biz/reset-password.html?lp=ODU3dHhjMXo%253D&lu=bW9qb2phbmU%253D
Clicking on it sends you to the test site reset password page. The link in the address line:
http://www.my-test-domain.biz/reset-password.html?lp=ODU3dHhjMXo%253D&lu=bW9qb2phbmU%253D
When forwarding the email out of the SquirrelMail account to another account the result is the same - in each case the reset password page appears (and it's blank) but does not provide redirect to the login page. Have tried the link in Chrome, Firefox, Opera.
Rather than using SquirrelMail I went into Horde (this is taking place through cpanel that I'm switching email clients) have the same link as above and clicking on it takes me correctly to the reset password page with the correct redirection message and available link which works.
The associated domain is a test domain I use to try out various MODX features before I attempt them on development sites that I own. It's a rip/reload location used to try things on fresh installs, so if any MODX'rs would like to use it to take a look I'd be happy to give you access. Just PM me.