Update:
I spoke to our nanny this morning about the situation, I said it as though it was something ds had told me and that I wanted to clarify if it was true. She described the situation exactly as the nursery worker had recounted it, so I don't think there was anything malicious or intentionally neglectful in her behaviour. I think it just genuinely hadn't occurred to her that it wasn't safe to do this. My gut feeling is that cultural differences do come in to play here. She's from a very rural part of Poland, she said it was only for 5 minutes and from her expression and demeanour she did look genuinely contrite.
I said that ds wasn't to be left on his own at, I said I thought she should keep personal shopping to when she didn't have the children, but if she was in town and did need to pop in to a shop I did understand that sometimes you just need to do a quick personal errand. I said if ds was egging her to let him go in to a toy shop she was totally allowed to say no to him and if he ran off she had our permission to carry out suitable punishments (no cartoons or removal of favourite toy). The funny thing was she did say that he was very confident at talking to strangers, and I said this was the very reason why he's not to be left on his own, he's not the type to run away from strangers, he's much more likely to chat to them, and if he did cross paths with someone with bad intentions he may even willingly go with someone.
I've given it much much thought over the past couple of days... I'm going to keep a very close eye on her. This is the last chance for her. One more thing and she'll have to go. I'm going to start putting out the feelers to see if there's anyone out there who would work for that kind of money. My mum has said she's going to keep a closer eye on her too, she only lives 15 minutes away and has for various reasons not been able to be around much.
Befuzzled yes it was where you think it was, upstairs. Ds was in ELC, she was in New Look across the way. You're right about the price of nurseries too. I do remember looking at nurseries when I was going back to work and I do remember it being more than the £100 per day we were paying our then nanny (that was net and it was killing us but bloody hell she was an amazing nanny!) Plus of course we'd have to pull ds out of preschool which I think would break his heart, he's got so many friends there. I know they get over it and make new friends etc etc, but it would have to be a last resort for me. I've got 9 months until he's at full time school :o
Thanks so much for the input everyone, I have taken on board everything that's been said. My gut instinct is that for now I will continue with our current nanny, but I will keep a much closer eye on her from here on. Sorry for mega post!