Applying for a visa for Congo (Kinshasa)

I'm a UK resident planning on visiting the Congo on a single entry visa and I've been told by the Congolese embassy in London that I will need a 'legalised' letter of invitation when applying for a visa. I have the letter of invitation from the organisation hosting my trip but my hosts have never before been asked for such a letter to be legalised. Can anyone help with how to go about getting this letter legalised?

Many thanks,
Chloe.