On replacing my Brake lines with MX5 CENTRE BRAIDED LINES i have found the rear hard line nuts to be rounded off and frozen on by a previous fool. The bloody Front was FINE!!!

Can i use Vice grips to try and undo them??
Can you buy replacement hard lines??
Should i just leave the old lines on there and take it to a Brake Centre and get them to replace the connections??
Anyone idea on cost for this?
Any Help much appreciated..