May be ordered through the nearest Family History Center. Searching the index of volumes 1-20 and 22 is free at Ancestry.com. You must be a member to view pages.
Oxford Dictionary of National Biography Full online subscription service (£)
FHL film numbers
Vol. 51 http://books.google.com/books?id=HskUAAAAYAAJ&pg=PA238&lpg=PA238&dq=illegitimate+children+of+Robert,+4th+Lord+Sempill&source=bl&ots=2RmC3vU1ne&sig=8vAAB5R8DNCtY0wgFkomHONw4Do&hl=en&sa=X&ei=btqBUvvVF8fs2QWoh4HoBQ&ved=0CFUQ6AEwBQ#v=onepage&q=illegitimate%20children%20of%20Robert%2C%204th%20Lord%20Sempill&f=false