Callitriche stagnalis?

I don't, but I recommend asking Paul Champion at NIWA in New Zealand.

Please check Rotala rotundifoloia, which is known from China

It looks like Bacopa monnieri, Waterhyssop, Plantaginaceae ( formerly in  Scrophulariaceae). Wait until it will flower.

Considering the morphology of leaves and habitat characters it looks like Bacopa species. Pictures of flowers may be needed for the further authentication.

Can be Lythraceae: Rotala rotundifolia (Buch.-Ham. ex Roxb.) Koehne.

It looks like some Rotala species (Lythraceae)

 May be B. monnieri syn. Herpestis monniera but correct identification can be done only after examining the flowers.

I think this plant belong to Rotala genus, very probably R. rotundifolia, not is Bacopa monnieri. Callitriche I don´t think so.

It looks like Bacopa monnieri, Waterhyssop, Plantaginaceae

You can check with Bacopea or Rotula

Dear All

Thanks very much for all your useful suggestions.

The top two genera contenders are Rotala (probably most likely) and Bacopa.

We are trying to get the mystery plant to flower and I will report back when I find out more.

Dear All

Our mystery plant has just decided to flower. It is indeed Rotala rotundifolia, which was suggested by a number of contributors.

Thanks to all of you for your help and interest.