I pronounce all versions of the name Al-ess-ter (non-rhotic), but I don't hit the t as sharply as I might for
Hester or
Martin et. al.
Preferred spelling
Alastair though.
I think
Alasdair is a touch more specific, regionally Scottish, rather than generally British
Daphne,
Rufus,
Theodora, and
Barnaby. I think if they were in a sibset all together a
Flora,
Marjorie,
Hamish, or
Walter would even it out. Individually paired with
Alasdair, they all work.