Becks - salary and conditions sound like a very good deal. You don't say how old your child is? I would be reluctant to leave an au-pair alone in charge of a child below school age.
You say DH works from home, if he intends to get any work done this still means she will be on her on child for extended periods of time.
The younger the child, the more experienced childcarer you need. Get someone who is already in the country, has worked for UK families before and your can follow-up references by speaking to old families on the telephone.
It may sound harsh, but ignore any one or agencies that try and send you girls with loads of experience in their own country. Unless it is an English speaking country (or unless you can sopeak their language and understand their system) you will find it very difficult to verify this, and it is not unknown for girls and agencies to embelish the truth in order to get the job.
You need to see them face-to-face to interview them. This also gives you a good idea of their English language skills, unless you are happy with your child quickly picking up and using broken English? You should also ask them what they would do in certain situations - ie. getting your child to eat vegtables (if they don't like them), or getting them out of the park playground when they don't want to go. There are no completely right or wrong answers - what you are looking for is their maturity and experience. Can they use persuasion, do they have any idea of what kind of persuasion is acceptable (do you want bribery with sweets everyday?), or appropriate with a child of that age (what will work with a three year old, won't work with a six year old).
Needless to say, you don't want someone who thinks it is OK to shout or use physical punishment or bully your child into doing things.
Give them an emergency situation (but don't call it that) and ask them what they would do? Say child runs into the road, or child falls off swing and bangs their head on the concrete, passing out for a minute of two? Or child had a very high fever?
If any or all of the answers sound like 'I would ask the father as he is in the house' this is bad news as it shows she has no idea of how to cope in an emergency, and in an emergency every second might count - and let's face it - your DH might not be there every second of the day, or an emergency could happen when she is out of the house with your child. Would you really want someone who would wait till they get home and chat with you DH before calling an ambulance?..............
In addition, girls familiar with working for families in the UK are also more likely to be familiar with things like how to work microwave, dishwasher, washing machine ect.
You may think it only takes five minutes to show someone how to use these appliances, but remember you are the one paying the bill when because they are not used to these things they 'forget' your instructions and leave something in microwave for 3 minutes instead of 30 seconds and it goes up in flames, or when they 'mistakenly' put delicates on a boil wash.
You need to think about what she will feed your child too, and what experience she has of preparing food. Will you child be living on chicken nuggets because that is all she can do. What is her food hygeine like? Does she automatically wash her hand before preparing food, or do you have to prompt her? Which means when you are not around to prompt it won't happen. Ditto with her making sure your child's hands are clean before eating too.
Don't forget to have a clear idea of how you expect someone to behave in your home. Do you expect her to eat with the family or are you happy for her to raid the fridge whenever she feels like it and take food to her room? Can she help herself to your booze? When her friends come round is it fine to open your best wine, or drink the last beers in the fridge?
Are boyfriends allowed to stay overnight, during the day, when you are there, when you are not there?
Smoking (what about her friends that might smoke?), phone calls, use of the car, what time should she be in at night - or are you happy for her to roll home at 4am after raving?
What do you expect from her before 8am and after 6pm, and on her days off? You need to be clear about this. On her days off is it OK for her to treat the place like a hotel - ie. leave her breakfast things on the table, ignore over-flowing rubbish bin, sit in the garden relaxing watching you sweating while you unload food shopping from the car?
Anyway - all or most of these questions you can ask her old families about. They will give you a good idea of her childcareing abilities and her personality - what she was like to live with.
This might sound negative - it's not meant to be. I have successfully used au-pairs on and off for seven years, and these are some of the pitfall I have picked up through bitter experience! Most of the girls I have used have been great - but you need to be really clear on how you see it working.
PS. The salary is good - I would lover it to £85 for the first 3 months then increase it to £100 if everything is satisfactory. This gives you both a break-clause too, and an opportunity to straighten things out, if needed, early on in the relationship.
Good luck