Lack of black tip on the bill is not a major problem. Moult is complex, as noted in 'Terns' and Adrians bird might not be fully adult (maybe a 2nd summer ?) due to darker lesser covert bar and a brownish primary covert, but the outer primaries doeīs look blacker than the inner one. According to 'Terns' Saunderīs moult later than Little (variable), and normally retains thre outer primaries which are deeper black comp. to Little. Number of black outer primaries is a variable character, and as Th_SQ mentioned a combination of characters would of course be wise. That head pattern on a 'clean' Saunderīs I would like to see! Intergrades complicates the situation further.
Check headpattern, males and females on Little here (from Northern Europe)