House Render - what’s the best?

Render doesn't really offer much in the way of insulation to a wall. Depending on the age and construction of the property determines how well insulated the external walls are. Newly built homes have insulation inserted during construction, post war homes tend to have a cavity that can now receive injected insulation but older buildings with solid walls have limited thermal properties whatsoever. These properties relied on breathable lime plasters and airflow because if moisture is trapped in the brickwork then it will make the brickwork cold whereas a dry brick will retain a certain amount of heat. Beware of just covering and trapping moisture. Lesson Over Choose what type of look you want then decide to use an external insulation product or an interior solution. Either or, its more or less the same process

Installing the correct rockwall insulation boards to external of property will give you so much more heat inside

You might not need to replace the whole of the render. Is there any signs of damp on the interior? If there is you could hack off the cracked area. If the render has blown it will come off easily enough and that patch can then be re-rendered. If there are no signs of damp indoors you might want to hold off as render will not improve internal insulation or warmth.