Hi KatyW. One thing our Nanny has been very good at is getting out and about with our two and also doing lots of activities. I would be disappointed if she sat and read a magazine while they played although of course accept that she will make personal phone calls and do a bit of shopping and so on during work hours.
I would certainly expect one outing a day - generally just a trip to the park or the local library but also, on occassions things like swimming lessons, crechendo/tumble tots, art classes, music group or general play groups. Once a week I think it is reasonable to expect an outing further afield (i.e. musuem, boat trip, farm etc). I would also expect her to build up a network of local Nannies and Mums so your boy has lots of children his own age to play with. Generally our children go to someone else's house once a week for tea and they came to us once a week (our Nanny works a 4 day week).
Most Nannies i know are very good at the arty side of things. Again, I would say that ours do finger painting and so on at least once a week and they also always make Easter cards, Birthday Cards and Christmas Cards together. In my view you should definitely try and encourage her to get involved in cooking with your son.
I am hopeless at trying to get a message across to my Nanny but if I were you, I would do something along the lines of saying that somebody has recommended a great playgroup or whatever nearby and it sounds like just the sort of thing your son would love. I would also buy a load of Art materials, children's cook books and aprons and so on and tell her that you painted / cooked together at the weekend and how much he enjoyed it.
It is also pretty standard for Nannies to keep a diary detailing their day (although ours tends to be full of the lyrics of the Bunny song from Music Group rather than useful info such as whether they've eaten or slept much!). I think the standard working week in London for nannies is 50 hours so your day isn't exceptionally long.
Nannies are lucky in the sense that they don't have to do any housework and so on so I think its reasonable to expect an interesting range of activities to be provided.