It would seem from what you have said in your op that you simply swapped your bed for the one at her house. 'I always assumed' gets you nowhere. Did she realise you intended to get it back? What were the terms for the storage of the bed? Would you have still wanted it back if a 'houseguest' had p!ssed in it? or if you had visited and your child had thrown up in it?
Unless you specificly and unequivicly said 'this is my bed and you are just looking after it and will have to return it to me when required' or words to that effect, then I don't see what you can do.
For her to have taken advice on the matter would indicate that there is more to this than simply returning stored goods. Anyway just because it is in her house doesn't make it her property.. Has somene told her possesion is 9/10 of the law or something? Tell that to the bailif or pc when they cme round to collect stolen good lol