BmPilot21, that's why you should never get (or indeed action!) a clearance 'descend two four zero zero (2400) feet' - it must ALWAYS include the words "descend to altitude" to avoid such fatal confusion. Likewise, the word "to" should never be included in clearance to a FL.
Another bug-bear - too many lazy people who don't confirm their SID details when first talking to APP/DEP e.g. "ABC123, passing 1000 feet, Bovingdon 9Z, cleared 6000 feet."
There was an AIC (140/1999, Pink 205) on the subject, dunno if it has been superceded. Think that some European ATC do things as per IACO (UK has filed an exemption?), i.e. no easy differentation between "to altitude" or "to FL" etc!