Hmm, ok. I've now pulled that out and put the test back in.

This does feel a messy though, and datapoint creation should probably be revisited at some point to try and tidy all of this up. In addition with this change parameter matching for single-valued datapoints can't be done on runtime type, since we decide whether to use the source before the method is actually run, so I've had to remove one of the other tests added in this pull request. That could potentially be fixed with a big restructure of how values are supplied and then assigned to parameters, but that's probably too big to look at now.