I've been trying to work out why I have such a poor credit score on Experian. They've given me poor to fair (550) but I can't tell if I'm actually in poor credit or whether they are trying to make me purchase their improvement services 
To make things clear I have never had a loan or mortgage. I have an outstanding overdraft of about £2,000 but it is still within the student interest free period. I also have an overdraft of £1,500 that I pay the regular interest payments on.
I have a couple of finance contracts such as a mobile contract and they all get paid on time and in full. I have no credit cards and a reasonable full time salary.
Can anyone explain? I've only just graduated and I feel worried about a poor credit score as I would like to start saving towards a mortgage. At the minute I live at home and my mum has lived here for years. She has excellent credit yet we still get letters through the post and debt collectors round from a previous tenant 8 years ago.
Is my address blacklisted?