This solution seems to work. Manually setting the response headers for any file in the directory /app_plugins/. Obviously best to remove once all backend development complete but when a backend is still developing as people using it is seems to do the job. Could be changed to apply to a specific plugin directory. Hope it helps someone.

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {

        app.UseStaticFiles(new StaticFileOptions()
        {

            OnPrepareResponse = (ctx) =>
            {
                if (ctx.File.PhysicalPath.Contains(@"\App_Plugins\"))
                {
                    var headers = ctx.Context.Response.GetTypedHeaders();
                    headers.CacheControl = new Microsoft.Net.Http.Headers.CacheControlHeaderValue
                    {
                        Public = true,
                        MaxAge = TimeSpan.FromSeconds(0)
                    };
                }
            }
        });

}