Overall I have had a good experience with the regulator and their support previously.
I am going to get some new Co2 tubing and report back to see if it works.
It will, either pick some up from your local shop or order a couple of metres from eBay.
The diffuser is fine, it is the weird Dong Ling tubing that is the issue. Normal co2 tubing fits the Twinstar diffusers perfectly.