hello, you should use a course sand and hydraulic lime(nhl 3.5) of a ratio of 5 parts sand to 2 parts lime, make sure the old pointing is cut out to a mininum of 25 ml, brush out all dust from open joints, before repointing wet all open joints, repoint using a finger trowel, wait till pointing mix is firm but not to hard, and beat back using a churn brush, a bit more advice, if the sun is out, once repointed keep the lime moist using water sprayer or GENTLY sprinkle with a hose.

presume at that age it will be a lime mortar rather than cement
you can try a company called conserv. take a sample of the existing mortar in a match box and send to them they will then match it up for you. REMEMBER TO INCLUDE A COVER NOTE WITH YOUR E-MAIL DETAILS
the address to send the sample to is
stone tech [ CLEVELAND ]ltd
lee road
bolckow industrial estate,
grangetown,
middlesborough,
cleveland,
TS6 7EB. TEL-01642 430099
there web site is www.coserv.co.uk
lime mortar does not contain any cement

hope this is of help

Try Trad-lime, they supply ready mixed lime mortar, various colours, you can buy it in bulk bags or large tubs.
If you want to mix your own, you could try a mix of 5/7 of soft yellow builders sand plus one cement and one lime, if you use snowcrete instead of blue circle/portland cement, you will have a more lighter mix.
The mortar doesnt want to be stronger than the stone.

I would use something along the lines of 5 parts sand, 1 part hydrated lime, and half cement. Hope that helps. Cheers