Don't whatever happens buy a local mayonnaise. Yes, it may be cheaper but it will also be quite ghastly and sweet.
Pay the extra and plump for Hellman's if you can find it or Kraft, it's definitely worth it. I made a cracking egg mayo last night, which even wifey was complimentary over. She's not a huge fan of the sandwich filling but was rather impressed.
The secret ingredient, apart from decent mayo? Add a good pinch of chilli powder and a dash of fish sauce. It sounds gross but trust me, it works.