Our clematis montana Rubens was badly attacked by aphids last year, and this
year it has flowered poorly/hardly at all, to the extent that it is now
over. I assume the two are connected. It needs pruning as it's getting a bit
big for its position. Question is, if I prune it hard back to the main stem
which is trained across the back of the house, will this -

A help with flowering next year, and
B help disrupt the aphid cycle?

And should I prune it now or later in the year?

TIA
Steve