>Nice one Viv I'll give it a go

Two other options (which actually bring nothing to the table - but I had to try them):

list.ForEach(delegate(Info i) { if (i.Postcode.StartsWith("HR"))  i.SomeProperty = true;  });

Or, more reusable:

 private static void MarkPostCode(Info i, string s)
        {
            if (i.Postcode.StartsWith(s)) i.SomeProperty = true; ;
        }
       // used with:
       list.ForEach(i=>  MarkPostCode(i,"HR"));