I can answer my own question and help other perhaps.
The black housing and chrome comes off as one piece, it is held in by two types of clips, metal spring clips and plastic tabs, gentle prising and use of a small screwdriver releases it.
You then have access to the corner parking sensor, the parking space sensor and the fog light to change the bulb.
Note they seem to be different part numbers. 1st pic is corner parking sensor, 2nd pic is the parking space sensor.
Searching the internet seems to return few if any cheap replacements(£70-90) each ! how or why water seems to get in no idea....mine plays up after heavy rain, for now i have cleaned it up and applied a little silicon grease around the seal in an attempt to seal it better.