Simon,
Create static routes to those 1200’s to use one of your static IP addresses as the preferred source:
/ip route
add dst-address=123.45.67.89 preferred-source=98.76.54.32 gateway=pppoe-outNow, when connecting to the 1200’s at work, the connection will come from whichever static IP you choose to use.
You say you have 5 static IPs… is this 5 seperate /32 address assignments, or is this a full /29 subnet that’s being routed to your dynamic IP?
If you have a /29 subnet being routed to you, there is a way to use all 8 addresses. Check out this thread.
Good luck!