How do I create a Web Service Endpoint for Template Items so I can update an Attribute called PHOTOURL via the REST api?

I have successfully updated the value for an attribute called PHOTOURL on a Stock Item via the REST API using the Default Endpoint.  I did this using Postman and also wrote my own standalone app which works without problems.  I am now trying to do the same for a Template Item.  As there seems to be no Default Endpoint set up for Template Items I created my own.  This works fine for updating the description field but fails when I try to update the Attribute value for AttributeID PhotoURL.  I am using the same method as I used to update Stock Items and have recreated the same Attributes Entity in my own Stock Items endpoint as a test.  The same body text that works to update the Attribute using the Default StockItem endpoint doesn’t update using my endpoint.

As far as I can tell I have set it up in the same way. Does anyone have experience of setting these up and can shed some light on where I’m going wrong?

See images below:

Thanks for any help,

Phil