You can aggregate multiple layer-2 links in a Portchannel. This way if one link fails the other one continue to forward traffic and no need to for STP blocking. For your Portchannel you can use LACP or PAGP if they are Cisco switches. If they are not Cisco you can use LACP.
Thanks for the reply, I did think of that but the service provider would need to configure port channel on their switches too, and into the same switch stack their end, this wouldn't be possible I'm afraid, so I've have a loop again.