There was a previous thread about transport out of Conques but the links seem to have changed. I found this info on the Conques tourist office site
http://www.tourisme-conques.fr/fr/venir-conques.phpthat may help:
Autocar (Bus) Line Rodez- Saint-Cyprien- Conques- Grand-Vabre
Departs: Mon 7:15 am (school term) Tues, Wed, Thurs Fri 7.45 am (school term).
Mon to Fri 8.05 school hols.
Length of journey ~1 hour 15mins, Cost 7.40 Euro
There does not appear to be this bus option on weekends.
Transport on demand: This appears to be a minibus that connects with the trains leaving from Saint-Christophe Vallon for Paris. This Saint-Christophe is not on the GR- it is 23km from Conques. This transport runs to connect with trains on the Paris line every day
on reservation, 24 hours in advance from 1 April until 4 November 2012, according to the website. The fare is usually 15 Euro, but it is 18 Euro at night, on Sundays and holidays. There appears to be a Tourist Office in Estaing, so maybe they would help you reserve this if you wanted to do it before you reached Conques.
Margaret