sorry we are not British, but a good place to get your visa for Nigeria is Libreville/Gabon.
We had a faxed invitation, carnets, 15000 CFA (30 Usd) each and got the visa next day. very friendly and helpful. we are German and Swiss and I know it can be different with a British passport.
Entering Nigeria, the imigration officer also wanted to reduce the visa - but I just said that the embassy issued it for one month and that we need one month. he tried to make a problem because it was a buisness visa, but again I said the embassy stuff knows what they are doing and that we had an invitation from a bike club, it worked again and we could enter for one month without paying a cent or a neira.
greetings from marocco, and good luck
mika and damaris