[bitfolk] apt-get slowing down enormously

It's on this PC rather than on a server, but apt-get has become incredibly slow recently. Reading the package lists is taking many minutes - using time to see how long 'sudo apt-get update' takes reckons it's Fetched 115 kB in 6s (17.7 kB/s) Reading package lists... Done real 28m27.642s user 0m19.944s sys 0m7.828s The root partition with /var/lib/dpkg is on an SSD drive, which has 80+GiB free space and is regularly 'trimmed'. There's nothing using an enormous amount of CPU or disk access. Even large applications are quick to start. Any ideas as to why this is happening? Ian