My distant recollection is that drivers on national rail were 80%
ASLEF, 20% RMT but the ratio was reversed on the Underground with 80%
of drivers RMT, 20% ASLEF. The ASLEF drivers were concentrated on the
Northern Line and one other (which I cannot remember).

Is this approximately the position now? If so, why are TfL expecting
significant disruption on all lines? Surely they are operating a
reduced service anyway for a Bank Holiday?