Shipping Options Problem when logged in...

Hi - I have a shipping options problem which I can't get to the bottom of and I hope someone can help.

Randomly, when a UK based customer is logged in and enters their shipping address, when they get to Delivery Method in the checkout process, it says 'Warning: No Shipping options are available. Please contact us for assistance!'
However, if the same person enters the same info as a guest, there is no problem and shipping options are shown.

I am using 2.3.0.2 and the standard Royal Mail extension.

Many thanks

check these first
viewtopic.php?t=50946
viewtopic.php?t=207440
viewtopic.php?t=153465
viewtopic.php?t=202895

I have checked the forums you listed and I can confirm that the weights are all setup correctly and the geozones are also correct.
I've also tried disabling and enabling the extensions and this didn't work.