I have had a number of bad experience with multigang sockets, beginning with one that cause my HiFi to 'hum'. Upon dismantling, it turned out there was meant to be a bar linking all the earth terminals. But that earth bar was completely absent, anything plugged in appeared to work, but was unearthed.
That first experience dates from 30 years ago, purchased at Woolworths (fond memories
) But ever since, I always buy top branded names, and I always dismantle them for inspection prior to use. You'd be amazed how often minor - but dangerous - faults are present, such as loose screw terminals that could lead to arcing.
You should definitely consider talking to Trading Standards. The switchboard at your local council will probably try to fob off with 'Consumer Direct', which is a centralised organisation that helps people to write letters of complaint, etc., which is not really what you want here. In my experience, if you insist that you want to talk to a local officer, they will put you through and will take a great deal of interest in something like this...
The only awkwardness with Trading Standards is you should really be talking to the office whose territory covers the Trader who sold the goods. If you didn't buy it locally, or not sure where it came from, there may not be much they can do.