Hi Milly,
First-off, your LL should have put your deposit into Deposit protection scheme, reagrds of when that came into force. Also, did you sign any photographic schedule or condition when you started the tenancy? Or did you at the end of the tenancy, take photos to prove you left if in a good, clean condition?
I rent 3 properties and with all I take photographs for the tenant and myself to sign, so that we can all agree that the property needs to be left in the same condition as shown at start of tenancy.
I would reccommend that you speak to a solicitor. Have you kept copies of the letters you sent to her?
Take legal advise - she should not be keeping your deposit without good reason.
Best wishes,
muchlovex