Problems putting new repo into Synaptic in Mepis

I have used Synaptic in various distros (notably Ubuntu) over the last two years and never had any serious issue getting a new repo accepted.
However, in M7, I had major problems here in that whenever I added the repo, any repo, to /etc/apt/sources.list I would inevitably get an error message once I had opened synaptic again (look here in the Mepis forum where more detail is provided).
In the end, it turns out that in Mepis you need to just put up with that error message and then hit Reload or apt-get update which will rectify things.