It worked.
I created a htl-maiser Pegasus Mail account on both servers, edited the mercury.ini file on the HTL server so that it contained:
[Maiser]
Maiser: HTL-Maiser # 'Username' of mail server account
Helpfile: C:\MERCURY\Mercury\MAISER.HLP # Help sent on failed maiser requests
Lookupfile: C:\MERCURY\Mercury\MAISER.LKP # Format file for the 'lookup' command
Send_dir: C:\MERCURY\Mercury\SENDABLE # Directory for the 'send' command
Logfile: C:\MERCURY\Logs\Maiser\~y-~m-~d.LOG # Mail server log file
Notify: C:\MERCURY\Mercury\TMP # Dir. for pending 'notify' commands
NoList: N # If 'Y', disable the 'list' command
Local_only: Y # If 'Y', local 'notify' commands only
and replaced every instance of 'maiser' with 'htl-maiser' in the *.mer files in the root of the Mercury installation folder.
All mail for these lists is now accepted by the APS server. The HTL server polls the pop3 box and collects the mail for htl-maiser, and all introductory emails sent to subscribers after they ahve successfully joined a list contain the correct htl-maiser@heritagelincs.org address.
Hoorah!