Went into my local home brew shop at the weekend to buy a new kit (Cwtch) and took the opportunity to chat to the guys in there about my home brew twang issue.
I've made three kits so far - Wherry, St Peter's Ruby Red and Festival Golden Stag and all have an element of twang which you can both smell and taste.
It was at the point that they asked me "How long is the beer in the fermenting vessel for?" and I answered "Three weeks", that they both jumped back and said "That's your problem - you're getting oxidisation in the FV". They were both in absolutely no doubt.
It varies from beer to beer, but for the Cwtch they reckoned on about 10 days in the FV. I'm going to start the kit soon and do exactly as they've suggested. If I manage to get a kit with no twang, then we may have cracked it.
I'll report back.
Also hoping to hear from reddibaggie soon (nudge, nudge!) in the Festival Golden Stag thread in the review section, who is running a test based on the effect (if any) of pouring boiling water onto the extract when starting the brew.