Thanks very much everyone 
- some quite different opinions here, but very useful to know what people really think. Some peoples thoughts have been similar to mine - maybe Samuel a bit religious or slightly 90's, William could be considered a bit dull/the Willy thing a problem, perhaps Xan is too uncommon a nn, but Alexander too long a name for daily use 
Obviously I like all three longer versions of the names (possibly the sound of the full Samuel the least) but I'm aware that many 3/4 syllable names get shortened anyway so would likely introduce him as the shortened version - especially for Xan since it's less common, but possibly for Sam and Will as well.
I agree that most people will call him whatever he is introduced as, if it's done from very young - but that he himself could decide otherwise as soon as he's old enough and I'd obviously respect that (whether he chose Alex, Liam or the full Samuel for instance).
I must admit I'm surprised to see the dislike for Xan (thanks to the few who said they like it/don't mind it btw!). To be honest though, I'm so contrary it's making me keener on the name 
. To me it's just a further shortening of Xander, which thanks to Buffy has become trendy/seen as "normal" as Alex over the last 20 years, as well as Xan being a name in it's own right elsewhere in the world.
Here's me thinking I'd done the safer thing by picking classic names in the Top 50 - I originally had "out there"/"try hard"/"pretentious" names like Bram, Orion, Atticus and Orlando on my list and decided I wasn't brave enough! 