Becky, I'm afraid that such an instrument does not exist. The only one I am aware of is this:
http://rsw.sagepub.com/content/22/2/219.short
However, having looked at the items, these might not be suitable for cognitively disabled respondents. Problem is that resilience is quite a complex and sophisticated issue.
That was my suspicion from my literature search, but I wanted to see if anybody was using anything that perhaps hasn't been widely written about, or that they have developed themselves in response to a lack of suitable commercially available instruments. Many thanks for your response, and the scale you mention is definitely one of the more useful/comprehensive ones I have seen.