This is fairly normal and the easiest way is an angle grinder (115mm) to take off the heads of the nuts inside the hull. (be carefull!!). Just punch out the remaining studs.
If proper anode studs have been used, it is not necessary to renew the studs each time the anode is replaced. The studs are fitted from inside and have a square flange to form a seal.
Thanks for your replies. Still not sure of method to remove. I think there is a tool called a nut cracker or similar name. Has anyone used one and to what success?
John, You can use a nut splitter if you have room, but an angle ginder would also do the job, if the nuts can be got at with that, but be very careful, try and shield everything from the sparks generated, have a fire extigiusher, with a spare body and wear eye protection. The nut splitter is painless and no sparks etc. Halfords or any good tool shop should have one, they are cheap nowadays and you will use it again, I garauntee! Plus yopu can lend it to your mates, for beer!