I found a work-around. It appears to be a DNS problem. Go to System Preferences>Network>Advanced then click on the DNS tab. Under DNS Servers I deleted my routers address and typed in the Google Public DNS 8.8.8.8 and 8.8.4.4. Restarted mail and it doesn't lose connection again. It worked for both my macbooks.
I'm no techy, but I assume this is just some sort of work-around that keeps the communication between the router and computer open. I read about it over a number of pages listed here (https://discussions.apple.com/thread/4268728?start=90&tstart=0). Again, the problem was only between my macbooks and this particular router. There must be a reason why they weren't getting along, but I have no real explanation as to why.
I hope that helps others.
Cheers, Damien.