Cool, that lamp really suits this!

Is it meant to run on a transformer ? Why the special plug ?

No, this will just be mains. The plug likely dates the fitting to a time when the UK still used 15A round-pin plugs, in the days before our current 13A versions. The plug you see here is a tough plug - there was a matching socket, also moulded in rubber. When plugged together, the combination also provided a degree of protection if used outdoors in a wet environment. They did offer a similar version in 13A years later.

Shows it's age a bit then, a great find

Now i just need the matching socket and wire that up to a standard plug to get this in action

I have assumed that moulded plugs are 70's onwards thing, have they been around so far back ?

So, this is moulded in so far as the rubber has been formed into that shape, but the plug is actually rewireable, there are a couple of screws between the pins and you can pull it apart to rewire it. So, very different from the moulded, fixed plugs things come with today