And following on from the teflon thread a while back, I was wondering
about the HSE implications of using this stuff, since it does eventually
blacken and burn away

Stuff on mine looks like teflon tape, but how the hell do you stick it
down?

If yours is burning it must be heating well beyond the temp needed for
bag sealing. Reducing the power a little may solve that. May mean it
takes slightly longer per seal.

Teflon is stable up to about 370 C, when it decomposes it produces HF which
is very nasty indeed. My guess is that the black deposits are partially
oxidised polythene. I'm not sure what the baking sheet stuff is (silicone?)
but it must take over 200 C. It won't be quite as non-stick but it would be
quick and easy to do a trial.