Eyes on Epsom, again

As things go from bad to worse for John Banks, the PM must be wondering how long he can defend him (and keep the ACT party's one pro-government vote in the House) before the association with Banks starts to have negative electoral consequences for National.
New allegations (reported here) by a former employee of Kim Dotcom suggest that, despite efforts to hide the provenance of the latter's donations, John Banks must have known about them, as he allegedly called to say thank you soon after the deposits were made.
Should that story be true, then surely John Key would have to sack Banks as a minister. Key has already made 'ethics' stretch a mile for Banks, and that has been damaging enough for National.
If stood down or sacked as minister, Banks could conceivably stay on as a backbencher and continue to give his vote to the government. But then there's a possibility, given the allegations against him, that he may have to leave Parliament altogether, forcing a by-election in Epsom.
Speculation about a by-election has already begun, with Rodney Hide (the former ACT leader and MP for Epsom) ruling himself out, and Colin Craig, the leader of the Conservative Party, saying he'd be in.
The latter, although true blue, appears to have so many policy differences with the National Party that he would not necessarily be a very helpful partner in government from Key's point of view.
Key can probably hold on to John Banks until the Budget legislation is through (for which he needs Banks's vote), but after that all bets are surely off.
After a dream run in their last term in government, National has suffered a series of discrediting blows since last year's election. And their relationship with ACT has been at the heart of some of these problems. We can expect some restructuring to the right of the political spectrum before the next election.