As far as I am aware, you can use them to offset all elements of the cost of employing an approved home childcarer / approved nanny (nannies gross pay and employers NI). Best to contact HMRC Tax Credits Helpline for advice though.
With 80% award, you will still need to pay 20% yourself, so I would be surprised if none of that 20% was taken up with paying employers NI (unless you are paying below the NI threshold, in which case there is no employers NI to pay).
You may wish to have a savings fund, to cover the eventuality of Tax Credits not paying you in time for when you need to pay your nanny. Delays I expect can occur and I would doubt a nanny would be very happy if they were not paid, due to some problem with tax credits.
What tax credit in particular are you able to claim - I presume you refer to Childcare Element of Working Tax Credit, is that right? There is a Maximum weekly amount for Childcare Element, I think it is £175 if one child, £300 if 2 or more children.
If you have one child, then with an entitlement of 80%, that would £140 a week cost that could be claimed. See WTC5
P49 Paying Someone For The First Time - this is a handy guide for doing the first week/month payroll for a new employee (in your case, a nanny). CA38 contains NIC's table A, which most people use. Employers NI kicks in once you are paying your employee £98 per week (all figures it should be noted are for 2007/08 tax year).
Lets say that your employee is paid £175 per week, gross - so that we can easily work out the 80% figure for tax credits. Tax credits would pay £140, leaving you to cover £35 plus employers NI which is £10.05 - so costing you £45.05 per week.
I think for tax credits purposes, you must be paying out (in some way) £175 for childcare in order that you get 80% of that paid. So for a useful figure, we want Weekly Earning + Employers NI = £175. Closest I can get is £166 + £8.90, £174.90 which isn't enough. So need to increase up to £167 + £9.02 = £176.02 which meets the 'spending £175 or over' amount we need.
So how many hours per week do you need a nanny for? Can their pay be £167 per week?