Hi Stuart,
I'm Stuart Halliday in WL too - Deans.

Our water is very soft - KH 1, GH 1 with a pH of 6.8.
So any nitrifying bacteria you use will really struggle to grow in it.
You need to get the water parameters more like KH ~6, GH ~4 with a pH above 7.
Also your bacteria products may not contain the right species of bacteria.
Regarding existing ammonia, there are numerous ammonia removing liquid products on the market.
Prime and
Amquel Plus are the two best.
But note, they (apart from Amquel Plus) don't actually remove it, only make it non-toxic for 24 hours.
There are of course
Zeolite stones available too. most shops sell this.
My tap water doesn't contain ammonia. So I think you'll need to phone your local water board up and complain. It could be faulty.
You can phone me if necessary - 01506 411162
Biomature contains no bacteria, but it does contain elements that may have increased the KH/GH/pH of your tank. Can you measure these 3 parameters in your tank and lets me know?