access squential counting

I have been asked by a client to convert an excel spread sheet to access this I have done but the unique identifier they have used is not sequential it has rows missing .... I need to be able to maintain this numbering with sequential numbers HOW? I need to set the default value of the field to the max id +1 can someone help with the syntax and where do I put it ideally i would like to put it in the table default value.