OK, I give up.
According to the manual for v3.0 L2TP-server has a keepalive-timeout setting, but in V3.30 I can't find it.
PPTP server set-up has a windows to set Keepalive, but L2TP doesn't. You can set idle-timeout in ppp profile, but not keepalive.
Can anyone give me a clue as to the obscure and convoluted means MT in its wisdom has decided to deal with this?