Hi Martin and thanks for the quick reply.
Indeed, that was my first thought so I had already visited http://ie.microsoft.com/testdrive/Performance/FishBowl/ and maxed out the number of fish. (I should have mentioned that already).
If I look at Process Explorer I can see the GPU at 70-80%, but it...
I'm using v4.60 on a Lenovo Z70-80 laptop with an Nvidia GeForce GT 840M card.
In HWiNFO, the entry for the GPU is blank with no readings.
However, using Process Explorer (https://technet.microsoft.com/en-us/sysinternals/bb896653.aspx), it shows the correct GPU usage.