This is what the DSCdecoder help file says about this channel:
"The Vessel Traffic Service (VTS) is superimposed on the VHF DSC channel and is used for the supervision of congested shipping areas."
"DSC and VTS use VHF channel 70 in the VHF marine band. ATIS messages precede all voice transmissions in the marine band in regions where the system is in use."
So it was VTS but still an interesting catch and good to know we can decode these. I've occasionally heard them, but this was the first one I've managed to decode, and at least we now know what they are.