Complete the parameterXMLString code re #8398

Still to be tested

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Begin unit test of new property re #8398

Unit test present just tests that it executes without error.
Also some fixes to ensure it executes without error.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>