In the worst case scenario I was prepared to buy one new ( as luckily I don't have to replace the Depth Sounder ). All of a sudden, not one but two turned up within a matter or hours, the same model - Plastimo Mini Contest.
This was perfect, the only question was would they go for silly money?
Well to cut a long story short, I managed to get one for a reasonable price, knowing that it wasn't perfect.
It arrived quickly, but the first observation was that 2 of the mounting bolts were missing and one was broken off in the unit.

After turning the bolt, it turns out it's a actually threaded rod... and not only that I have some stainless bolts that are the same thread.

Hacksaw to the rescue..

Drilled out the stuck bolt and now there are 4 mounting bolts where they should be :)

Next unscrewed the light as it wasn't working...

Bulb totally blown, but it's the same size as a 5mm LED, so a quick search of t'bay later and for the princely sum of £1.42 I have a 12v LED installed.