It has to be something so at this stage I feel it would be wrong to make any assumption. At the beginning I would not have expected there to be a problem with that combination and there may well not be but all things have to be considered in view of what has happened.

What next?

I really doubt that this is going to work from this position and I think it would be a waste of time just waiting any longer.
If it were me I would make a 100% water change without using prime. Then add a single dose of biomature and test the water for ammonia and see if that one dose is converted to nitrite and then nitrate without adding a second dose.

Would not using Prime kill off the bacteria, if there is any?

No not really, there is very little chlorine actually gets through the system to the tap. Lots of well known breeders and fishkeepers never even dechlorinate. I always do as a precaution but there is little point in repeating the same thing which has just failed and because the ammonia seems so stable there doesn't appear to be that much of a bacterial colony present anyway.
So its worth the risk.

Ok, so I have done a change, but silly Jade forgot to empty the Eheim canister?!

I have it running now, and did a test anyway. I have added no BM yet.

The Ammonia is 0.5ppm
The Nitrite is 0.25ppm

So nitrite has reappeared already? That's interesting because there must be some nitrifyers present after all. Lets hope the ammonia drops now.

I have added no BM yet. Should I?

So those levels are residual from the filter? Anyway, yes add just a single dose. Then test the levels and wait and see what happens.

Ok I added one dose. I have tested half hour later.

Ammonia is 2ppm
Nitrite is about 0.25/0.5ppm

Allowing some error as not natural light.

Do you think I over dosed? The instructions say that when either the ammonia or nitrite reaches 5-10ppm, to stop adding. Maybe I should've stopped sooner..

If the ammonia is just 2ppm and it can go up to 5 - 10ppm how is that an overdose?

Anyway don't add any more lets just wait and see what happens with this one dose.

Nah I mean before the change

I shall leave it for now. Thanks for helping :)