Well, after some much needed advice from the forums I did a 30l water change tonight. I couldn't do any more at this time as my RO unit isn't fast enough

I also trimmed all the plants and found a couple of the cabomba's hadn't rooted and instead the lower stem had rotted in the substrate, I've trimmed and replanted those that where salvagable and removed the others.

After all this water readings where identical to yesterday. Still at least the tank doesn't stink any more and it looks pretty. In the mean time I have more RO on the go and will do another 30L change tomorrow if the ammonia is still too high.