UV light is actually not visible to the human eye so if it was working you would not be able to tell - to test it you need to shine it over something that reacts to it like fluorescent material (from a highlighter pen for instance)
NB its also not safe to look at directly with the naked eye sunglasses are recommended and only then for a very short period of time (a matter of seconds)
Thanks. Actually visited my local shop at lunchtime, he showed me the same lamp in operation in a cabinet under a 500lt tank, there was an obvious glow from the transparent pipe leading from the unit - this may have been more visible in the darkness of the cabinet compared to where I was inspecting the lamp myself in daylight. He did say that I should check that the uv bulb hadn't worked loose in transit. Will do that and then take a chance and fit it at the weekend. Probably worrying about nothing.
Turned out that my trip to the shop was worthwhile, I took the back off the unit last night and the bulb was loose in its fitting, I reconnected the plugs at either end, turned it on for a few seconds and it glowed correctly.
So all ok now, I just have to fit it at the weekend.