You would be looking to pay anywhere in the region of £6-£11 an hr gross depending on nannies experience and qualifications and whether you would consider a nanny with own child.
You could search privately on sites such as www.netmums.co.uk, www.childcare.co.uk and www.gumtree.com
or use agencies www.kidsmatteruk.com are nationwide, sorry dont know any in south wales.
www.nannyjob.co.uk has a job section, a nanny cv section and also an agency search option so worth a look.
remember to ensure nanny has a CRB check and at least 2 checkable references (can you guarantee its not a friend giving them a reference - just a scenario I have heard of!)
Do you want them to have first aid quals, be a driver?
You may be able to find a nanny looking for first nanny job who has done a couple of years in a nursery and willing to work for £6 gross an hr as would a slightly more experienced nanny with own child.
Why not pop a job advert saying what you require in a nanny and at the end state willing to consider applications from those looking for first nanny job and/or nanny with own child.
state you are looking at paying £6 gross an hr and would like a min of 18mths-2yrs childcare experience (this could be nursery work, mothers help, extensive babysitting- although would want lots and lots if only experience) you may find some really fab applicants looking for someone to give them a chance.