I don't think you can get married in a marquee at your mum's, you woul dhave to use a licensed venue. Most Hotels are very expensive, the Register Office is your best bet.
Then you can all go back to the marquee in Mum's garden and have a few butties etc. You should allocate an amount per head, say �3, i could do a fab buffet for 50 people for �3 per head, check in the local newsagent who bakes cakes to sell from home, you only need a one tier cake really,.