  We calculate the double pole contribution to two to four fermion scattering
through $W^{\pm}$ currents at tree level in the Standard Model Effective Field
Theory (SMEFT). We assume all fermions to be massless, $\rm U(3)^5$ flavour and
$\rm CP$ symmetry. Using this result, we update the global constraint picture
on SMEFT parameters including LEPII data on these charged current processes,
and also include modifications to our fit procedure motivated by a companion
paper focused on $W^{\pm}$ mass extractions. The fit reported is now to 177
observables and emphasises the need for a consistent inclusion of theoretical
errors, and a consistent treatment of observables. Including charged current
data lifts the two-fold degeneracy previously encountered in LEP (and lower
energy) data, and allows us to set simultaneous constraints on 20 of 53 Wilson
coefficients in the SMEFT, consistent with our assumptions. This allows the
model independent inclusion of LEP data in SMEFT studies at LHC, which are
projected into the SMEFT in a consistent fashion. We show how stronger
constraints can be obtained by using some combinations of Wilson coefficients,
when making assumptions on the UV completion of the Standard Model, or in an
inconsistent analysis. We explain why strong bounds at the per-mille or
sub-per-mille level on some combinations of Wilson coefficients in the
Effective Lagrangian can be artificially enhanced in fits of this form in
detail. This explains some of the different claims present in the literature.
