The Mortgage Works are part of Nationwide BS, I did have a quick look at their website a few months ago and noticed they had quite hefty fees, think it was 2.5% of advance for BTL and 3.5% of advance for Self-certs.

Some other lender's have fixed fees rather than a percentage but you really need to compare the total amount payable including all fees, charges and interest over the term that you expect to stay with that lender. The number of BTL mortgages has reduced quite significantly over the last 18 months so the choices probably won't be that great. I never paid ridiculous extra fees like that but I was using lifetime trackers, now it seems they all carry fees.

To find a mortgage broker just google for BTL Mortgage brokers ****** (town you live in). As far as I know required deposits recently were a minimum of 25% with a rent coverage of 125% or 130% of the interest payment.