After several weeks wait, I now have confirmation from Esri UK, via Esri Inc, that this is a bug as it's not possible to populate the SelectFeatureTypesPage using the SDK.
The reference I was give - if it's of use to anyone else was:
BUG-000102430 : Unable to populate SelectFeatureTypePage using the ArcGIS Runtime for Windows Mobile
An Esri UKs conclusion was:
The FeatureTypeListControl is what is used to populate the SelectFeatureTypePage. The FeatureTypeListControl is populated by FeatureSourceInfos. FeatureSourceInfos is a read only property, which means the FeatureTypeListControl cannot be populated. This in turn means the SelectFeatureTypePage cannot be populated.
As the product is now in mature support and shortly to be retired, Esri Inc will not provide a fix. In any case, due to our time constraints, we had already worked around the issue by creating a custom SelectFeatureTypesPage that allows for population of the feature type list with a collection of FeatureSourceInfos. We made this visually the same as the original and it works very well, although took several hours of development that we hadn't budgeted for.
I hope that helps anyone else who might be struggling with the same issue.
Regards
John