BS21 is a manual rotary switch based on the symbol. IL1, I would guess a temperature switch or controller of some sort. The rectangle plus the dashed line to the adjacent normally open contacts suggest a relay, with the rectangle being the coil, but in this case it probably represents power supply connections to that device. The diagonal line with the horizontal segment is similar to an NTC, and the output apparently controlling a compressor certainly suggests temperature control. The 'A B C' and 'D E F' blocks I guess would be control inputs, so that would fit with your suggestion that BS21 is switching temperature probes.
There are a handful of different styles/standards for symbols in wiring diagrams like this, plus sometimes people just make things up when there isn't a a suitable standard symbol. Sometimes it helps to take a guess and do an image search for '____ wiring symbol' or similar and see if anything looks like a match. Or sometimes you just have to trace out the device (those numbered connections can be a big help, if the wires are actually labeled), and see what the thing physically looks like