Edwardian outside window sill help

Hi,
I helped a friend strip the exterior sills of his house in Battersea and when we got the paint off with a heat gun it became clear that the stone or concrete sill needed repair.
One of the sills has 3 large cracks leaving a gaping 5-10mm gap (it has a little decorative feature at each end underneath and I think one of these is
holding one end up as it could drop without it.

How do we treat the crack? Do we just fill it with mortar..
Is in ok to use 'International 3 in 1 to seal prime and undercoat the sills?
before applying the weathershield gloss.

Thanks in advance

I'll try and get some photos..

Try plastic padding/ car body filler to fill the gaps up.. A tip I picked up on here once; when you fill. rub some sand ito the filler before it goes off and this will make it look more concrety than a smooth finish.
I don't know 3 in 1 stuff but bare concrete should have an alkyd resisting primer if your finishing with an oil based gloss.

good luck.

Thanks, it has worked!