  We reconsider the problem of finding $L$ consecutive down spins in the ground
state of the XY chain, a quantity known as the Emptiness Formation Probability.
Motivated by new developments in the asymptotics of Toeplitz determinants, we
show how the crossover between the critical and off-critical behaviour of the
Emptiness Formation Probability is exactly described by a $\tau$ function of a
Painlev\'e V equation. Following a recent proposal, we also provide a power
series expansion for the $\tau$ function in terms of irregular conformal blocks
of a Conformal Field Theory with central charge $c=1$. Our results are tested
against lattice numerical calculations, showing excellent agreement. We finally
rediscuss the free fermion case where the Emptiness Formation Probability is
characterized by a Gaussian decay for large $L$.
