House Martins or not

Hi All,  I was on an old stone bridge over the river Dee and saw lots of birds swooping around in the sky very like House Martins and yet I wasn't sure they were Martins.  Then I noticed they were nesting in holes in the bridge sides which is not identified on the RSPB site as a nesting place of House Martins. These birds seemed browner than Martins, any ideas?