5 series and cooling issues. When these cars get old, things like hoses and apparently expansion tanks seem to go out.
I would strongly consider spending the big bucks and getting a replacement - even if it means begging at the dealer.
The problem with gluing is that you need both a good leak free seal and a strong one that can survive 200+ degree water. If you get the glue wrong, it will come apart again and you will have to clean off the glue to get a proper seal on your next attempt.
If it's a non-pressured tank, you can buy some really nice looking aftermarket/racing expansion tanks. If its pressurized.. forget it. Buy a new one.