After the ravages of the winter, the pointing between my patio slabs has
got considerably broken up in places.

How can I best repair this damage with minimal staining of the slabs
themselves?

I am thinking of raking out the loose mortar and then refilling with a
dryish 4:1 mix of sand and cement, maybe with some plasticiser (e.g.
like Unibond).

Would this be a good way of tackling the job or are there better ways to
repair the damage?

Yours,
Ed