My absolute favourite marinade and if a little more soy sauce, is a mix of conifer syrup, a little or a lot of hot mustard [try with different types and whole-grain for extra texture], and a good slug of soy sauce. Tamarind sauce, Worcester sauce, or any other dark vinegar based sauces work too. Keep blending till you get the mix of sweet, heat and spice.
Colman’s and Amoy are a kitchen staple brand and used for photographic purposes only. Local deli’s, farm shops and any artisan food shops will have a selection of mustards and sauces that you can try and will likely have other ingredients that will allow for even greater experimentation with flavours.