Ok, I played a bit more with the machine over the weekend and I think I can answer my own question:
Firstly, this machine has a boiler not a thermoblock so the lack of pumping sound during steaming is normal.
Secondly, I googled as hard as I could and found that the steam wand knob and fixtures on these machines seem to clog up fairly easily. So I descaled again, this time allowing an hour or so for it to soak. This seemed to help quite a bit. To fully clean out the steam lines and fittings I carefully dismantled the plastic casing to get access to the steam knob. If you do this you'll see the fittings are fairly easy to remove and are serviceable. So i pulled out the knob internals and scrubbed the fine layer of whiteness off the fittings.
The steam works ok now. It's fairly week compared to a something like an em3800 or em6910 but that's to be expected for this model.