Hi. I can't find anything from the UK, but I found a set from Germany:
https://www.safferthal.de/de/VR-91-Riemen.html
It's also a good idea to clean the idler tyre that drives the loading ring. The ones for the VTC 5000/5150 or newer models won't fit as these have a completely different mechanism.
Regards,
Fivos
Update: There is a loading belt for the 5300 (same as the 5400) in the UK, right here:
https://wiltsgrovedev.co.uk/tv-satel...o-vtc5300.html