paul161201, Good evening.
Several things going on here.
1/. If the render is indeed blown and sounds Boss when tapped there is a reasonable possibility that the render does actually have cracks in it, OK you may not be able to actually see them, the cracks can at times be microscopic, this is down to the nature of Cement and how it breaks down and breaks up.
2/. The area of render that has De-bonded can at times be fairly small, that is a small "Roughly" circular in area can be tolerated, however once the area of detaching Render gets larger then there will probably only be one option.
3/. With respect, I note you live in Scotland, there are loads and loads of issues with failed and failing render on Ex-Local Authority properties all over Scotland, you see patchwork patching on gables and front and rear elevations of such properties, I believe that the Specification used by the Scottish Local Authorities was seriously flawed ?
4/. A Question? does your property have injected cavity wall insulation?? [of any type] If so then this can have a huge bearing on your problem, especially is the property is in a very exposed position. There is a burgeoning industry in insurance backed Repairs as regards Cavity Wall insulation, Nudge, Nudge??
5/. Now for your Question? replacing the render if Boss will need to be undertaken at some point, that point will depend on how large an area of render has blown, Any chance of you uploading a digital image? of the areas involved.
Ken