Limited knowledge here, but as I understand it they cannot withhold any pay unless you have agreed it, they can't even make a deduction from your wages as a re-imbursement (for instance I use the post at work for personal stuff, but they have to get my permission to make the deduction from my wages).
However, if you've verbally agreed then although a bit dicey they are ok, however, it's a verbal contract and if they have reneged on that agreement then they are in breach.
The awkward bit is that if you push the issue then you may force them into administration, and run the risk of not getting any money from the administrators anyway.
I'd be deeply suspicious of the 'we aren't going under' statement if they can't afford to pay the wage bill, ok, maybe someone else is holding out on them and it's drained the funds, but still, not a good sign. Start looking for a new job :(