Doh!
Not the first time I've made things more complicated than they need to be. These Literals aren't floats, they're just decimal numbers. XSD has a perfectly good datatype for these.
added to the graph in rdflib thus:
Literal(value, datatype=XSD.decimal)
returns from the AG store thus:
<typedLiteral datatype="http://www.w3.org/2001/XMLSchema#decimal">0.8049</typedLiteral>
sorry for the noise, jjc