I have resolved this issue by doing the following:
Create the RADIUS servers:
radius server ISE01
address ipv4 ###.###.###.### auth-port 1812 acct-port 1813
timeout 10
key ####
!
radius server ISE02
address ipv4 ###.###.###.### auth-port 1812 acct-port 1813
timeout 10
key ####
Now create a group with those servers in
aaa group server radius VPDN-NewISE-Radz
server name ISE01
server name ISE02
Now create an authentication list with that group in
aaa authentication ppp NewISERadzAuthList group VPDN-NewISE-Radz none
Now add this authentication list to the Virtual-Template
interface Virtual-Template3
ppp authentication chap NewISERadzAuthList