Around 1987 the copyright law was changed to give automatic copyright to the 'author' of a 'work'. Prior to this change, if you were commissioned to take pics the copyright was automatically assigned to the 'client' (whomever paid the bill).
Unless specifically agreed otherwise a photographer owns the copyright to his/her work. If you are PAYE for a company, i.e. a staff photographer rather than a freelance/self-employed, the company automatically owns copyright in any 'works' you produce.
At that time I was in my London studio and you should have seen the rapid output of new agreements being flung out by the magazines/agencies to try and get back the copyright. Most flatly tried to bully us by stating they wouldn't hire anyone that didn't agree to the new terms of contract for freelance work. So much for a change in the law!!
As far as I know, this is the current situation re copyright. No assignment to third-party, you keep copyright.
In this case, you should still own the copyright, the 'payment' wasn't agreed in advance (bad mistake) although unless the girl specifically stated that the chocs etc. were in lieu of the photos it probably constitutes a 'gift' and not remuneration for work done. The model agency certainly requires your written permission to use the shots although it is generally implied that when you have done a 'model folio shoot', the girl/agency can use the pics to promote the model (but not sell them for gain).
In future, I suggest you have a 'charge the going rate or do it for nothing' policy and make it clear that you retain copyright in ALL you work and that you specify what 'permission' you are giving before you do the job. Better still, send her to a professional
Regards
Les Meehan