Thanks for all your advice, both on the costs, and on the need to find someone with childcare experience and good communication for the sole care.
The requirement for an aupair is a bit odd for an 18month-old, I agree. It is to help cover a part-time job only, not full time.
The situation is that the youngest (will be 21 months in January, when new arrangements need to start) will start in a local nursery from shortly after his 2nd birthday. The nursery is good but only covers till 4pm.
So once the nursery place is available, we only really need afterschool care and (walking) school/nursery runs. So about 9 hours a week sole care. Plus holiday care for a mature 4 year old, and maybe a bit of housework/babysitting. But until then, we need 27 hours childcare to cover 3 days/week.
I suppose we need to be very clear in any communications about the two different regimes (the first is only sole charge childcare, the 2nd is lower sole charge, but a little bit of housework, babysitting)
As to alternatives, good childminders are in fairly short supply near me (has to be very local). I've also been considering employing someone more specialised on a short-term basis. But I'd rather the kids have fewer adjustments to make, and my income doesn't really stretch to temp nanny rates.
So that's where the requirement comes from really - I hope that makes more sense....