Alternative code, this time from the 1980s, and targeting the CMOS variants, from another W (Acorn's Sophie Wilson): http://8bs.com/basic/basic4-bycategory.htm#Menu - scroll down a bit to find the floating-point operations.

I think the format was: 1 bit sign, 7 bit exponent, 32 bit mantissa.

(This code is for the version of BBC BASIC that shipped with Acorn's Master 128.)