Spent a lovely day yesterday driving an old 1973 Triumph Stag- got back to my nice new (new to me anyway) Merc and was amazed to find that I couldn't open it. No response from remote - used the key and of course alarm went off. Thought I should check out the manual - it was locked in the glovebox, which does not seem to be able to be opened if central locking not working. Manual is now stored behind the passenger seat!! RAC (who amazingly arrived in less than 10 minutes) could find no reason for the flat battery- no drain showing on their system. Was all ok this morning after a 20 mile run last night and overnight park in hotel. As a complete numpty as far as these things go - should I replace the battery anyway to be safe- if so, any recommendations for type of battery? There is a huge variation in price... The existing battery is dated December 2006.