ugh....the lowest house in the area..I know that situation, my rental property next door to me has that same famous tag. I'm the first to know of a sewer problem, it backs up into my basement sink, first time it happened it backed up the second floor bathroom which overflowed down onto the brand new kitchen and trashed the ceiling (which was so new I literally hadn't paid for it yet, got the contractors bill the day it was trashed). Good luck fighting it, my township was very good and honest with me about the whole thing but there are laws to keep them from being liable for stuff like this, I had a long and heated discussion with their insurance company about it, the township didn't even know they weren't liable for this stuff until that situation. It gets worse if anything weird is happening when it's raining like you said, or during any kind of weather, then they get to use the "act of god" line and all bets are really off....that sewer could flow your house away and they aren't responsible for it

That is all info gathered within the last year here in PA, hopefully things are different down there, good luck with it. Oh and you may want to put your own pvc ball valve on your sewer line, if you notice something going on just shut that off and let them figure it out, it will at least keep you from having half the towns sewer in your basement.