stream drifting out of sync

[Ian Steele]

I've been trying to solve this issue for several days but just cant fix.
I have 2 Raspberry pi CM4 systems (both identical OS) locked togetherwith ptp4l. I have got a nice low-latency rtp stream sending 48k 24 bit audio from 1 to another.
I'm using the audiotestsrc in gstreamer to generate tone bursts so I can look at the signal on my scope  fromeach device and see the latency/sync.
The commands below seem to work well - no glitches and very low latency(approx 8 mS)
But on the receiver the latency gradually increases. It seems to go up by about 1mS per  3 minutes.
TRANSMIT:gst-launch-1.0 -vv audiotestsrc  is-live=true wave=ticks apply-tick-ramp=true tick-interval=100000000 freq=10000 volume=0.5 sine-periods-per-tick=100  ! audioconvert ! audio/x-raw, format=S24LE, rate=48000, channels=2 ! tee name=t ! audioconvert ! queue ! rtpL24pay ! udpsink host=239.255.0.10 t. ! queue ! alsasink
RECEIVE:gst-launch-1.0 udpsrc address=239.255.0.10 port=5004 multicast-iface=eth0 ! application/x-rtp, clock-rate=48000, channels=2 ! rtpjitterbuffer latency=5 ! rtpL24depay ! audioconvert ! alsasink  processing-deadline=0 latency-time=1 buffer-time=1
Do these look ok?Maybe I need to be clocking the receiver from the incoming stream somehow?
TIA

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