As Leo says, its the rebuild cost that you need. Remember that BUILDINGS are the structure that you cannot take with you. CONTENTS are what you can take away. One Insurer I dealt with said that you should imagine your house being turned upside down. Whatever falls is CONTENTS and whatever remains in situ is BUILDINGS.
'Old' is a subjective term. It could mean a terraced house built in 1800's or a Tudor house build in Tudor times!
Onus is on you to have the sums insured correct.
One suggestion, would be to ask your broker/insurer to recommend a sum insured (in writing). That way, should a disaster ensue, you might have the protection of the document to pursue wither insure or broker?????