ftp.uk.debian.org flaky?

One of my package builds last night (~22:30), and two in a row today 
(~12:55), failed at the "install build-dependencies" stage with

Writing extended state information...
Err http://ftp.uk.debian.org/debian sid/main amd64 python3-sqlalchemy 
all 1.2.15+ds1-1
   Cannot initiate the connection to ftp.uk.debian.org:80 
(2001:1b40:5600:ff80:f8ee::1). - connect (101: Network is unreachable) 
Could not connect to ftp.uk.debian.org:80 (78.129.164.123), connection 
timed out
E: Failed to fetch 
http://ftp.uk.debian.org/debian/pool/main/s/sqlalchemy/python3-sqlalchemy_1.2.15+ds1-1_all.deb: 
Cannot initiate the connection to ftp.uk.debian.org:80 
(2001:1b40:5600:ff80:f8ee::1). - connect (101: Network is unreachable) 
Could not connect to ftp.uk.debian.org:80 (78.129.164.123), connection 
timed out
E: Unable to fetch some packages; try '-o APT::Get::Fix-Missing=true' to 
continue with missing packages
Reading package lists...

Other builds succeeded in between, and in a browser the READMEs load but 
very slowly, suggesting it is not completely down.

Switching to ftp.fr.debian.org works, which suggests (but doesn't prove, 
given the intermittent nature of the problem) that it's not my end.