Thanks for the responses. It would be nice if the modem just issued smoke and sparks, because that would be nice and straightforward!! We borrowed another modem from someone using the same ISP, so it was just a case of plugging it in. According to the Speed Test our download speeds jumped to 1.2Mbps, real life download was about 500-800Kbps, and we could stream youtube and online TV at high speed. After an hour of this we figured we needed a new modem and swapped back to our crappy one. Guess what?!?
Our modem now does everything the test modem did!! We have had three computers streaming at the same time, upload and download at the same time, do basic downloads and browse - all seems to be maintaining the higher speeds. I rang the ISP and explained the testing procedure and results and they still think it's the modem - something to do with repowering. I'm at a slight loss because the Billion is still under warranty and we were going to speak to the supplier. Might still do. In the meantime we can get a free inhouse-designed modem from the ISP, so that's an option as well. Means we can do direct comparisons for no extra outlay and we would have two modems, just in case.
I didn't realise there was so much to modems. We will keep an eye on the current one and see how it goes, but if it does what it did before it will be a gradual decline in speed.