Andy,
Will be best to loosen the evaporator as well and partially remove the system so you can gain access and see what the issue is. The single carriage bolt has two nuts, one that holds the bolt in place and the other bolts down the compressor plate. There is a space as well, so don't loose that!
You'll have to remove the two screws holding the line cover in place as well.
Perhaps the Video Tutorial at the bottom of this page may help a little...
Vintage-Restorations Digital Workshop