It does seem daft that if you have an ME you cant carry over over to an ME. On my FAA licence when I did my ME i also got my Complex and High Performance endorsements signed off in my logbook, they carry back to the SE.
If JAA Land wants you to undertake differences training for someone with an ME rating then it should just be a case of undertaking the 1 hour check flight usually required by the club (hence cover the 'proficieny check' mentioned by whopity) and proving you can apply what you have learnt previously to the SE - 5 hours for someone who ALREADY holds an ME (and assuming not a complete muppet!), is just overkill.
J.