As OutragedFromLeeds says, MNT provide a course. The other provider I know who does it is BabyEm - distance learning, or at a London venue.
Ofsted will do a new CRB (now DBS) check as part of the registration process.
First Aid needs to be suitable of Ofsted registration, many courses are though do check before signing up to a course.
Sounds like the registration process may take your nanny a few months, depending on how soon they can get on courses. So if starting childcare vouchers in the new tax year (April) then you will need to stock pile them for a while. Check with your voucher scheme provider with regard to how long you can stockpile vouchers.
Childcare Voucher Savings Chart 2012/13, £600 a year... perhaps more, so not a huge saving, especially if you are paying for the Ofsted registration and training courses. So do see what saving you would make and what costs you would incur, plus check how being in a Salary Sacrifice scheme affects any company pension, state 2nd pension, SMP, mortgage application, salary related bonus, Life cover, company car, or anything else at work that is or may be based on salary as your salary will be REDUCED by being in a salary sacrifice scheme.