Help Abysmal 4G download speeds

This post may be more relevant to O2 users in the UK, but it might be a more general problem. Basically my 4G data download speed are rubbish even when I've supposedly got full reception bars indicated. I know that this can be a network congestion issue, but it seems to be so widespread that I'm beginning to wonder if there is something wrong with the device.

I've started doing random speedtests via the Ookla app in different locations. With one or two exceptions my download speeds have been between 1 and 3 Mb per second, and often as low as 0.1Mb per sec (basically unusable).

Today I was indoors with 3 -4 bars of 4G reception. When I ran the test I got 3Mb. A short while later I noticed that the signal reception had changed to H+ and ran another test where I got 11Mb. I then went outside where the signal changed back to 4G, and with full reception I got a speeds of 0.8 and 2, a fraction of the speed obtainable under H+. It doesn't really make any sense

Before I start complaining to the network provider (and we all know how interested they will be!) are there any settings that I should look at in the phone?