I'm going through this EXACT same thing right now. I'm getting married in April also at a small chapel. I made up a separate invite and put this on it:

Together with our parents
We invite you to share in our happiness
As we celebrate our marriage at a reception to be held on
Saturday, the fifth of April
Two thousand and eight
Five o’clock in the evening

Place



I've seen in quite a few different places where people say do not do this as the guests will be "offended". Well if they are going to be "offended" then I don't want them there. If they can't be understanding as to why I could not invite them to the ceremony, as I did not have enough seating, then I don't want them there to help celebrate at all. With every wedding that I've ever been invited to, I've been honored to even be invited - EVEN IF IT WAS JUST FOR THE RECEPTION!

Hope this helps