ah i have done a little more digging and have stumbled across the Investigate -> Failures section within the application insights section of the portal.
initially loading this page displays "error displaying your content" with the following details so assumed this would not be able to help:
Error: Failed to retrieve the blade definition for 'AspNetOverviewV3' from the server. Manual require of the following modules failed; ["_generated/Blades/AspNetOverviewV3" : {Error: Couldn't load "_generated/Blades/AspNetOverviewV3" at "https://afd.hosting.portal.azure.net/appinsights/Content/Dynamic/iE-E1AhCnKyg.js" as the following dependencies failed to load; ["Overview/ViewModels/AspNetOverviewBlade" : {Error: Couldn't load "Overview/ViewModels/AspNetOverviewBlade" at "https://afd.hosting.portal.azure.net/appinsights/Content/Dynamic/5I1DBHoYhADo.js" as the following dependencies failed to load; ["FxInternal/Controls/ProgressBar" : {Error: Couldn't load "FxInternal/Controls/ProgressBar"; error code 503},"FxInternal/Controls/ProgressBar" : {Error: Couldn't load "FxInternal/Controls/ProgressBar"; error code 503}];
but the failures section does display the top 3 exception types and drilling down into this I can see a dependency of my dependency is null.
having added this extra dependency to my startup class my function is now behaving as expected.
i still did not see any output in the logs so would still like to know how to access these logs since the kudu webpage no longer appears to offer this option