Since then postgresql (on Debian at least) has added the option to
have multiple ‘clusters’ and multple versions running on the same
machine.

Fortunately the old version (7.4) was still available

Steps

• restore /var/lib/postgresql/data
• restore /etc/postgresql
• mkdir /var/lib/postgresql/7.4
• mv /var/lib/postgresql/data /var/lib/postgresql/7.4/
• chown -R postgres.postgres /var/lib/postgresql/7.4/
• pg_createcluster 7.4 data (this moves the old data into the new structure)
• pg_upgradecluster 7.4 data (upgrade the data to the latest postgres version installed)

Other issues: my old data was in a character set I didn’t have installed as I now use UTF8

I had to add en_GB ISO-8859-1 to /etc/locale.gen

and run locale-gen

But I now have access to my old data in both old and new server
versions (I’ll probably delete the old one but it might be useful for
testing)