Just to be clear here... on the furlough scheme you can still pay furloughed staff full pay, its the claim from the government thats restricted.
Please be aware any staff not working you can claim 2.5k a month for them.
Also, id look at legal advice re the contractor not paying. Can you stop your work and not be in breach of contract if they arent paying, send all staff home on furlough, claim the 2.5k per month each and you top them up yourself.
That would, based on what you've said, greatly help your cashflow.
Even if half your workforce could be furloughed. The scheme is there to help the employer. Its up to you if you top their wage up.
There is also a 12 month no payment loan backed by the government released yesterday that might help you with your short term cash flow a bit until the contractors get their head out of their ass and pay.
https://www.gov.uk/guidance/apply-for-a-coronavirus-bounce-back-loan
Cash flow should be your main concern and discussed with employees. I know as an employee id rather be furloughed on less money knowing I'll have a job to come back to than paid close to my normal wage for a few months then have my employer go bust and have no job...