Hi Frogman 84
"My questions were though: Is it ok to be running the amp at all with only one LM circuit in operation?"
I think Enzo has already answered this... not reccomended practice.
Would you drive a 4 cylinder car with only 2 working ?
Perhaps downhill to the local mechanic..if you get my drift.
You don't really want to run the risk of another burn out which could cause more destruction.
"Am i right in thinking that could be a factory fault or is the plastic strip supposed to be between heatsink and back of LM?"
I think you will find it is a mica washer to electrically insulate the back of the IC from the heatsink and to allow (with heat sink compound) thermal conduction to the heat sink.
Years ago I can remember mica being used in slow combustion stoves/wood heaters as windows to the fire.
It can take high temperatures but can easily flake and disintegrate.
These days silicon impregnated rubber (sili pads) are used as they are much less messy therefore eaiser to use in production..I'm still not convinced they are as thermally conductive as mica + heatsink compound but are sufficient in a lot of cases.
You must make sure all the surfaces are clean and perfectly flat and there are no bits of grit or solder particuarly metal shavings etc which could penetrate the mica or not allow good thermal transfer between
So yes the washer is supposed to be there.
I have found with the boards its eaiser to cut out the component then just remove the debris which won't damage the board as much as trying to remove it in one go.