Is your nanny going to be live-in or come to you on a daily basis?
Some things I can think of:
Home Insurance - check that it covers you for accidential damage by a visiter/employee.
(will obviously depend on your view of insurance as to if this is worth the bother)
Company Car Tax? - By providing your nanny with a car, will you be liable for company car tax? I'm not sure on that one, worth finding out. I know families who have a nannies and they get the nanny to provide their own car - mind you, they are live-out (Daily) nannies, rather than live-in.
Also with regard to insurance, cost of insuring a nanny under the age of 25 may mean that you go for a nanny aged 25+. Do contact your insurer for details of adding sufficient cover.
Food - Breakfast for nanny, tea/coffee/etc. Dinner for nanny. Most nannies would eat with the children - after all it's a social event and encourages discussion at the dinner table.
Fuel cost for trips/school runs.
Misc Expenses - Having a weekly kitty for misc expenses can be very handy for your nanny. Amount will depend on the children's ages, a budget of £10 per child per week would be suitable and can be used to pay for small expenses while out, toddler groups, drinks/ices, parking, attraction admission charge etc.
Not sure why you should be paying for your nannies training. You could of course pay for courses but you don't have to do so.
There are more and more nannies and nanny agencies these days accepting GROSS salary figures, though that may make little difference to your calculations.
Remember that if your employer will provide Childcare Vouchers, then you could use £50 a week of those to pay your nanny, and thus you could save over £800 a year on your tax/ni bill. Care-4.co.uk has a calculator which can help you see how much you could save.
Note: Your nanny needs to be an Approved Childcarer. Some nannies are going through the approval process.
Nice to see that you would consider a male nanny, do dislike it when people only think of nannies as being women. Over the years I've met some great male nursery nurses/nannies.