Systemate SP overflowing

Hi im a registered heating engineer, went to a systemate SP today that is constantly overflowing with hot water. could it be the mixing/blending/tmv valve for the dhw passing through the cold inlet into the cylinder? have had this before on normal OV DHW tanks, and following prior visits from a few engineers that have advised coils split etc etc I have repaired without replacing the cylinder just a tmv. Customer had advised no pressure loss on the heating so not convinced that the coil is split, have replaced the ball valve already so can only think that its this tmv that's filling the overflow with warm water. Heating and DHW all fine as far as customer can advise, overflow not really a problem but causing a nuisance with building manager. Any suggestions?