Pay wise you can pay anything from National Minimum Wage upwards. I would say probably a bit more than NMW given you are in London, so say £7 per hour Gross upwards.
A payroll company (List of companies on this message thread) will help produce payslips, calculate amounts payable and remind you when to transfer payments to HMRC.
As they have other income, you may need to give them a P46 (HMRC will supply various forms when you register as an employer) which they may complete stating that your job is their Second Job/Not their primary income. That would then probably result in the job being fully taxed (rather than using personal tax allowance). Should not overly complicate things, though is important that you agree the Gross salary, not take home salary (net).
Payroll Calculator will let you play with salary figures and tax codes to show you how much it will end up costing you, once Employers NI has been added on to the cost. For more info about taxation, see www.hmrc.gov.uk/PAYE
- What are the legal requirements? Are there any? I think she has to be CRB checked - how do we go about doing that? Are there any other requirements (first aid etc?)
There are no legal requirements at all regarding nannies. You won't be able to get a CRB check done, as that has to be done via an agency/regulator. She's a family member anyway, so you should know a lot about the person's history. Do you have any cause for concern?
First Aid would be desirable but many parents don't have first aid training. A good course for a childcarer or indeed a parent is Early Years First Aid.
- If she wanted to get a childcare qualification while doing this, what sort of courses are available (we'd pay for her to do it)
Cache Level 3 Diploma Childcare is the most suitable I'd say for various childcare jobs (rather than just nannying). Problem however is that the Diploma is 1009 hours at college and 750 hours at various placements. So not really possible at the same time as working.
There are some distance learning courses but I wouldn't recommend that route, as it can take a long time and not give the experience of working in various childcare settings.
NEC are offering the new Level 3 Childcare Course, details are a little sketchy though. For example it says that someone has to be working in a childcare setting... would nannying count? It may be possible... or it might not.