.On my hunt for the culprit I swapped over a few bits. One of them was a set of carbs I'd won off eBay for £10 Buy it Now. Under £20 posted.
They look close to brand new tbh. I hatched a plan to have a pair set up for economy (old'uns) & a set for track days (new'uns) & summer months.
The plan was still in the box in the shed till the above problem happened, so with the unknown plug issue forcing my hand I pulled the carbs off the bike & pulled the carbs from the shed.
I swapped over the 178 jet front & 180 rear (Keihin) from the old'uns & that was it. The new'uns kept Std needles+washer & #48 pilots with the pilot screws 2 1/4 out front & 2 1/2 rear.
Then I fixed the issue with the new plugs
I rebuilt the bike & took it for a shakedown 60 miles around the island with my mate.
It's now a bit rich low down, no choke required & a slight rich fumble at really low cruising speeds 40mph in top.Touch the choke (enricher) & it fumbles more but no problem really.
I did use Hawks setup to set up the new carbs, as I know it does work but I found it drinks fuel. It's ball park like he states & the bike loves it rich anyway.
What I've noticed is that with me leaning out the midrange with my needle filing in the old set, it had lost some mid range pull, but it did give me a increase in cruising mpg. Around 10mpg extra & at the time I was happy to live with it like that.
What I have never taken into account was the wear on the old set of carbs I had.
I knew the new set I'd put in was set up rich, but there isn't much soot on the tail pipe & doesn't reek of petrol either when running.
Not too sure of mpg but at the moment but I'm just enjoying the bike again & that lovely mid range.
You did good with a set of carbs for that price Phil 
