okay, found it... answer courtesy of Gawain Lynch (not an LQ member)
1. Get your correct key ID (do this anyway just to double check)
$ gpg --list-keys
You will get some output like this:
pub 1024D/5BECD197 2004-09-19 Reboot Kid <rebootkid@nateandamy.org>
sub 2048g/ACE9C67B 2004-09-19
The key ID you need is the one on the 'sub' line, i.e. ACE9C67B
Obviously yours will be different
2. Open evolution select Tools -> Settings
3. Select the mail account you want to update and click Edit
and go to the Security Tab
4. Fill the "PGP/GPG Key ID" field with 8 zeros (00000000)
5. Press OK
6. Repeat steps 3, 4 and 5 only this time paste/transcribe the sub key
ID in to the "PGP/GPG Key ID" field