Tax
Speak to the tax office in the first instance, ask for their advice if the tax bill is large and you don't have the funds and they should be able to give you an idea of options. You need to find an accountant/bookkeeper (my parents use a bookkeeper to do their tax forms for them, but he was highly experienced and recommended) as soon as possible so you know what kind of payment you are actually looking at. I would get your husband to ask people who work in the same field as him or who you know are self employed who they use, a good one always gets recommended. Plus as others have said, make sure you have receipts for everything, it's amazing how much you can claim back on, plus you can retrospectively claim back VAT for one year if your accountant suggests becoming VAT registered, this may also help with getting some cash flow.
MOT
Wouldn't check for electrical faults, little come back concerning the MOT I would think.
Car warrenty
I would take it back to where you got it and kick off. Research everything you can first and enquire with trading standards, if you ring they may give you a pointer as to whether you are on the right lines with what you think you can ask for.
Cash flow
There are lots of options available to you. Much as you don't want to use credit cards, provided that you have a good credit rating and know how to use them they can be very useful. For example 0% deals for a year on spending could help with your every day expenses and free up some capital now (assuming you can catch up within a year to repay the spending-only a good idea if capital is desperately needed now). You can even get a card from Egg that allows you to transfer cash to your bank account to the credit card and pay this back at a life time rate of around 8.9%. They don't have to be bad news, you just have to be strict. However, if you are nervous only use them if you are confident as to how they will be repaid and after you've spoken to a proper debt counsellor on one of those charity lines, most definately NOT one associated with a debt consolidation service (you pay them for arranging payment plans that are not necessarily in your best interest and are something you can sort yourself-without a fee of £50 per month).
Make sure you have added up all of your debts and commitments, incomings, interest rates that are applicable before you ring, they can offer much more help that way and scary as it seems, seeing it all written down actually makes it better when you have a plan of how to clear it. You can prioritise your debt and work out where to put it much easier if you know what it is first. The unknown is far scarier than reality when it comes to debt, trust me x (former spendthrift extrordinaire and reformed spendaholic).
The good thing is you know you are in a pickle and are trying to do something about it. However bad things seems, they are not as bad as you think, honestly. You are being positive and sorting stuff before you start missing payments ect, which is normally when people start to spiral down.
Hopefully a bit of good financial advice and some frugal planning should have you back in the black sooner than you think.