Thanks dolphin - I cannot find the P5 on the airmar website so do not know if it is what I need.
Actually, I got a bit confused with the part numbers.
I saw the ad. in both PBO and YM for the EM transducer, part no. DST900-EM, and as my boat is ashore for the winter thought I might change the transducer to a non paddle wheel type in order to avoid having to withdraw the impeller every time (a favourite home for all kinds of little sea creatures if its left in), with the usual ingress of water and subsequent clean up.
Went on the airmar website and identified an ultrasonic transducer, (non paddle wheel of course), part no. CS4500; contacted jgtech (very helpful) and they have already quoted for this part, which is VERY expensive. Now I have asked them if they can supply the new EM transducer. That's the story so far.