Just for fun, I've calculated the Banzhof power index for all of the political parties in the UK parliament. This gives their relative power in terms of being able to swing votes.
Sinn Féin 5.9%
Plaid Cymru 4.3%
Sinn Féin don't take up their seats, so with them excluded the figures are:
Not sure what it shows, except that without the coalition the actual voting power of Labour and the Liberal Democrats would have been absolutely identical, despite the very different number of seats.