Read the manual for the device(s) feeding into that tundish... it'll likely tell you the most likely cause (in part depending on whether water hot or cold).
If an unvented HW cylinder a G3 registered plumber may well be required. Some, however, might have a user-replenishable air bubble that, if it has been absorbed, can raise a safety valve and leak water into the dish when the pressure rises enough. That might be user-fixable, following the user manual instructions.
Show us a picture of the HW cylinder (if that's what it is) that is connected to the tundish, including any label on the front detailing the make/model/size etc
Your unvented cylinder does not have an internal bubble therefore there are no user related operations that can be performed. You need a qualified unvented engineer in to check your system over and service it. Part of that will be to check and check the pre-charge of the expansion vessel.
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