Corporation tax for non-profits

We are a non-profit organization, working in overseas development and conservation and registered as a company limited by guarantee in England since 2005.   We have just received the CT04 Corporation Tax review form and I would just like some advice on filling it out. We have a small reserve fund but do not make any profit, and rely on donations from charitable trust funds and private individuals.  All our staff are volunteers. I assumed that we would not be liable for Corporation Tax, but I just wanted to know what to expect from HMRC when I return the form.

Thanks, Chris