Hi Nightshade
I recently was 'donated' a bat detector (Mk 3 Batbox) by a 'batty' friend and like you wanted to record the echolocation calls emitted by the bats.
I couldn't think how to do it without carrying around some huge tape recorder which isn't practical whilst roaming around woods etc in the dark. I mentioned the difficulties of recording the sounds to my brother and he produced a dictaphone that he no longer used. It works!!!!
I am now the proud owner of a Phillips Executive pocket memo 491 and it so far has recorded
Noctule's, natterer's, and
Pipistrelle's (45) and (55).
Unfortunately I haven't yet managed to find a lead to fit the recorder to my bat box
I probably will need an adaptor, but so far just holding the dictaphone near the batbox is proving okay for taping the sounds.
You can also get 30min or 60 min tapes for this particular recorder so I can record and keep my efforts!!
Hope this helps
Tragus