Although email marketing can work it doesn't really give you good value for money in my opinion. As fisixc points out, to do email marketing legit can be quite expensive. I can think of 1001 things that £7,000 could be better spent on.
Also keep in mind the damage it could do. You are going to be sending emails to people who don't know you. These people have probably mostly clicked on a checkbox on a site somewhere that says "can we send you emails promoting 3rd party companies" - so long as you are using opt-in. If I got your marketing email though, that would be it you'd be off the role call of companies I would spend money with. People don't have respect for companies that do this kind of thing. I would spend your money on enhancing your rep. on an on-going basis rather than a quick email punt in the hope of a bit of short-term business.