I questioned my isp as a result of getting slow downloads.
Did a test via speedtest.net before downloading, and during downloading.
The result was 5.1mbps before and 1,4mbps during.

The answer I received 8 days (they promise 8 hours) later is as follows:-

Speed test only show the current available speed
if you do a speed test during a download you will only get what is left
Please bare in mind slow download can come from the server you connect
to
Can anybody out there confirm whether this is a valid answer or not?

To me it looks like they are throttling despite their denial thereof.