Dont give in. I had a great nanny, who started with me when my DD was 3 months old. Sole charge, working 6.30 - 5.30
As a live in I paid her £350 a week, but after a year she decided she wanted to live out so she could meet more people. As I was desperate to keep her I agreed and also agreed to her £500 p/w salary increase.
Anyway in Feb, she decided she wanted to stop being a nanny and try another career.
I was gutted as she had been my DD main carer for so long, and I had done everything I could to make her happy and keep her. This wasnt enough for her. But, when I started looking I found the talent and nanny pool greater than when I had originally searched 1.5yrs previously. I found a brilliant replacement within a week.
My new nanny lives in (much better for my husband and I who both work long hours), is amazing with my DD, who now at 22mths says please, thank you and sorry without being prompted. She also never stops working, I literally have to force her to sit down and take a break.
My ex nanny? Well she has realised that unfortunately everything I said about the job market wasnt me scare-mongering, but the truth. She is now in the unfortunate position of having to return to London and find a job as a nanny, at a time when all my friends across all industry sectors are losing jobs, taking pay cuts and certainly in no position to pay top whack for childcare.
So to the OP I say, take a chance and you may find someone better than your current nanny - as amazing as you think she is.
Good Luck.