Unfortunately the OP is in the UK and metronidazole is not available here without a prescription. A vet may be willing to supply a prescription, though there aren't many that deal with fish.
Is the Seachem MetroPlex not available in fish stores that carry Seachem products? Or can it be ordered online and smailed...who would know? Just thinking outloud, as this is one of the very rare medications I will use without worrying. When this issue happened to me, the biologist said it would not hurt to medicate the food of all new fish acquisitions in the QT for a week, if I wanted peace of mind since internal protozoan have no external signs until it is too late, and they do seem to be more prevalent at least in NA.