Douglas Carswell has proposed an excellent reform to further keep the executive in check...
Here's a simple idea. Require every Whitehall department - and each associated quango - to have it's budget annually ratified by the relevant House of Commons select committee. No approval, no money.
Now that really would be a fundamental shift in power. It would also bind the hands of the opposition parties more firmly, since their fingerprints would be over the governments spending plans, either in approval or disapproval. It would require opposition parties to have firm principles over public spending in the long term. However it could present an unfortunate side affect of binding parties to such an extent that HM Opposition could not present a radically divergent spending programme at the election because of this.
It would, though, fundamentally weaken the executive, forcing it to justify every £1 it taxes and spends. And it would help empower Parliament - changing the composition of Parliament would mean something.
It strikes me, however, that this reform would make most sense under a system where the executive is directly elected.