How Safe is this 5-way ceiling lighting DIY install
Is the 10amp chocbloc sufficient for the load of 5* 42w halogen lights?


Yes, 5 x 42 W = 210 W, 210 W / 240 V = 0.875 A


  Is the 'manual' twist of 5*neutral 'blue' cabling from each arm safe twisted into a single 10amp chocblok connector?


Five is a lot. I'm surprised the fitting didn't provide a better way to unite them. The main issue is ensuring that the connection is tight and that enough of all wires are clamped under the screw. If in doubt you can use multi-way connectors such as Wago push-fit or lever-clamp connectors which have a separate position for each wire.




  Can I increase the risk by double insulating say the single Brown arm combine to live supply?


I'm sure you mean decrease the risk but I don't understand what you mean by double insulating.

For general lighting applications there isn't any need to apply insulating tape so long as everything is done correctly. 

The live and neutral wires are adequately insulated and you shouldn't trim more than the minimum insulation needed to fully insert the wires into the connector. After doing so, there should be no exposed copper, the screw heads in the connector are protected by the plastic within which they are recessed, there should be no danger of metal to metal contact.

Any metalwork in the light fitting that can conceivably come into contact with people should be connected to protective earth.