Where a grant is convenanted to a company/charity for a number of years while they carry on particular activities - is there a minimum number of years that this needs to be in place?

At the end on the covenanted period the company/charity can write off the amount given in their books - is this correct?

What type of legal agreement will cover this - a simple line stating the terms and conditions of the grant - will that do it? 

I am arranging a general meeting for our 33 consortium member village halls and would like to arrange for a speaker to come and explain about Gift Aid to the halls. Do you know if anyone offers this service - I would appreciate any contacts who would be willing to attend our meeting which will be based in Teesdale in County Durham. Thank you.