Hi linakpl - just come across your thread - interesting as I have just posted a thread today on the same subject, but different bag! - my old Roxanne has a weak magnet too - I guess this can sometimes happen over time, but I am presently in a bit of a wrangle with Mulberry repairs regarding whether it can be replaced or not - mine is the old style of magnet - are the ones on Alexa the newer, bevelled ones?
Haven't had any response yet to my thread - so wondering if this is something that doesn't happen on a regular basis - maybe we are just unlucky - but, if they are the newer style magnets, you should be able to get it sent off (as it isn't under warranty you'll prob have to pay about £15/20 to get it done) - but good luck, and let me know what happens!!