I used no more nails and it worked a treat, my arches are rusted too which is way it came off...
Before you stick it back on get some rust treatment on there to stop the rust creeping up the arch, I spoke to the spanner man when I picked my car up yesterday and he said take the arches off, cut the arch back a bit & get some rust treatment in there or grease to stop the rust spreading and then no more nails to stick the arches back on (thanks spanner man I knew that one!)