I'll have another go at removing it.
When it was unscrewed it would only come out of the tank by about 1". So I could see the small main supply pipe (not the reserve pipe as I called it previously) and I could rotate the tap in the tank but it seemed to be firmly fixed otherwise. Then I saw the edge of a plastic sleeve just peering out of the tank and it seemed to be this which was stopping the tap from coming right out.
I didn't want to force it and risk having the little pipe break off in the tank.
But out its got to come................