First thing to check as you say only TTS locos are affected. Is your track power connection. The Hornby TTS decoder is very sensitive to this, far more so than other decoders.
Double check your track power connectors and make sure that they are either R8241 or R8242 [DCC Friendly]. If they are R602 or R8206 or R8201 then they will have a DC Analogue suppression capacitor in them and are not suitable for DCC.
The capacitor if present distorts the DCC data signal. All decoders are affected to some extent by this signal distortion, but TTS decoders are affected more severely than others. The distortion can cause a wide varied number of control anomalies. One of which is 'DC Runaway' where the decoder switches into DC mode and all DCC control is lost [which is what you are reporting on the TTS locos]. So even if you do or do not find any track connector capacitors, check the value of CV29 and edit it if needed to disable 'DC Operation' support. Use the online CV29 calculator in the link below to show you how to understand the working of CV29
Open up all of your track connectors and look for and remove any capacitors you find.
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See also – further TIPs on how to get the best user experience from this forum.