  We derive general constraints on the mixing of heavy Seesaw neutrinos with
the SM fields from a global fit to present flavour and electroweak precision
data. We explore and compare both a completely general scenario, where the
heavy neutrinos are integrated out without any further assumption, and the more
constrained case were only 3 additional heavy states are considered. The latter
assumption implies non-trivial correlations in order to reproduce the correct
neutrino masses and mixings as observed by oscillation data and thus some
qualitative differences can be found with the more general scenario. The
relevant processes analyzed in the global fit include searches for Lepton
Flavour Violating (LFV) decays, probes of the universality of weak
interactions, CKM unitarity bounds and electroweak precision data. In
particular, a comparative and detailed study of the present and future
sensitivity of the different LFV experiments is performed. We find a mild
$1-2\sigma$ preference for non-zero heavy neutrino mixing of order 0.03-0.04 in
the electron and tau sectors. At the $2\sigma$ level we derive bounds on all
mixings ranging from 0.1 to 0.01 with the notable exception of the $e-\mu$
sector with a more stringent bound of 0.005 from the $\mu \to e \gamma$
process.
