Filling / pointing to stone mullion

[Bowness72]

Hi.... I need to fill a vertical straight joint between some existing stonework and a stone pillar / mullion. The gaps are approximately 3mm at one side and 10mm at the other. Ordinarily I'm aware that lime mortar should be used for stonework but as this is a very localised filling exercise mainly for prevention of rain ingress I'm considering using a weak sand/cement mortar possibly even a pre-mixed tub from usual diy places.

Does anyone think this would be a really bad idea?

Cheers

 

[catlad]

I once got a stone mastic for a similar job and it looked ok
when I filled it, other than that there are trowel
mastic's available.
Sand and cement might crack being that small !