On Friday, 20 April 2018 18:51:25 UTC+1, T i m wrote:
Hi all,

So, I have a couple of 6' single TL-D 70W tube fittings in the kitchen
and the other day one started flickering.

I had a box of new tubes so replaced them both (they were due anyway)
but one still wasn't right. I found a spare starter and replaced the
iffy one and all was well.

The resistance between the filiments is near infinity when "Off" and near zero when the tube is running.
This the basic problem.

The "choke"/inductors provide the high voltage jolt to establish the "arc" between the filiments and then limits the current when running (hence "choke").

Chokes are very reliable. If it goes wrong, you'll probably be able to smell it. (They can also go open circuit)
As someone else has said switch the thing on and remove the starter and observe. If it starts to flicker, the tube is at fault (may take some time).

Both tube and starter have sub-atmospheric gas filled glass bulbs. (Plus mercury in the tube) The common fault is for air to leak in.
The other fault is for a filiment in the tube to go open circuit.

A fluorescent tube with electronic "choke" is as efficient as an LED bulb.