have an old pre-frabricated garage, which is made up from pieces of
concreate. Over the years, it's started to leak through the gaps in
the concreate slabs which make up the walls. Instead of forking out 5
grand plus for a new garage, I was going to attempt to seal the walls
using an external waterproof sealant (Thomson waterproof concreate
sealant, etc) and for the gaps, something like UNIBOND waterproof PVA.
I was going to attempt to use a foam product (don't know which one)
but I think the gaps are too small.

If the concrete panels were fine for years, why would they now need
coating in waterseal?

If there are gaps, it seems evident thats the problem to solve. Cement
mortar, or a tough sealant would usually be the thing.

Expanding foam can expand with huge force, enough to being walls down.
NT