Answer

Our supplier states the temperature range to be -50C to 70C. However, the maximum temperature seems on the low side. The stoppers can certainly cope with being in boiling water (we have tested this), so I suspect the temperature range is on the cautious side and probably refers to sustained use for long periods within that temperature range. Shorter periods at higher temperatures are likely to be fine.

 Regarding autoclaving  they were less helpful and only suggested trying it for yourself!