But on the assumption that 1400 is now the limit for DF packets within Three's core, PMTU Discovery surely will cap at 1400, no? It should yes, but if that is temporary and becomes 1470, then why limit your modem? Let the system work it out.
In the 1990s and early 2000s we used to set the MTU on our PCs and laptops to 1400 because the PMTU didn't exist in Win98 and similar. There was always strong advice to never change the router, but to set the MTU on the PC/laptop.
Normal MTU discovery relies on ICMP, since so many devices block ICMP it quite often doesnt work, not a good idea to rely on it.
For this reason some OS's have a new TCP based variant which will work but usually is "disabled" by default. It also only fixes TCP not UDP.
e.g.here is the documentation for the linux tunable that controls it.
tcp_mtu_probing (integer; default: 0; since Linux 2.6.17):
This parameter controls TCP Packetization-Layer Path MTU Discovery. The following values may be assigned to the file:
1 Disabled by default, enabled when an ICMP black hole detected
2 Always enabled, use initial MSS of tcp_base_mss./quote]