Both methods of doing the speed test should give you the same result and be a good test, but ultimately it harks back to all online streaming using the same net connection.
@Chris1003 - you mentioned in your first post that you had no problem on other apps. Can you confirm what other apps you've tried?
If you've not already, try watching an OnDemand programme directly from the V6 EPG/menu - if it's a BBC programme it'll load the iPlayer app, any other broadcaster will use VM's VoD platform. If that works, then the issue is likely with the C4 app and it's worth trying the same programme from another device and/or connection. If that also has problems, then the issue is on the 'net connection from your V6 : which can then be troubleshooted bit-by-bit if necessary.
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