some simple rules to follow
1. The first contact will not under normal circumstances come vir email, there is no central database so it is next to imposable to trace.
2. The company should have a web page, listing there phone, and address
3. The phone number should be in the phone book
4. The address should not be a po. box
5. although there are foreign companies that work in the UK most don't so be careful if you are approached by an overseas company, I don't know any African firms.
6. if the Value sounds very large, Millions of pounds it is probably a scam.
7. Even if you have been approached by some one claiming to work for a genuine firm make sure the phone number and email address is the same as the one on there web page and in the phone book.
8. check out there registers, Data protection, companies house (Fraser & Fraser is a partnership so is not registered there),FSA, ISO.
9 do a simple google search and see it they come up any ware that they shouldn't, but make sure they do come up.
10 ask your solicitor.
11 finally as a last resort ask one of the other firms if they have heard of them, we all know each other.