Arthur 51 wrote:

Glueing seems like not a good idea. So can bog standard exterior
filler or
even car body filler be a better way of 'glueing' these joints?
Or something else?

I'd use polyurethane glue, it expands and foams when you apply it to gap
fill, but make sure the joints are clamped up as tightly as possible to
prevent it forcing them apart:

http://www.toolstation.com/shop/Adhe.../sd3198/p52489