I would GUESS they are 3/4 BSP, might have been easier to adapt 3/4 BSP to 28mm maybe. I'd leave them well alone if they are, as they have probably been there since the cauliflower was new and trying to get them off now could do more harm than good. plenty of 3/4 BSP to hose tails if you are connecting directly to hose or easy to convert to 15mm polypipe/speedfit/copper etc.
is the "cold in" not the connection on the other side with a bit of a plastic fitting still on it ? what you have labelled cold in is a drain off.