I recently purchased a car on a finance option but my Grandfather had to be a guarantor due to my age and lack of credit rating.
Once the direct debit was set up I contacted the finance company and changed the bank details to my account. Would this mean because I have kept up with the payments and it is coming out my bank account would I get the positive credit rating or would it be benefiting my Grandfathers credit rating? Thanks :-)

If the agreement was taken out in your name then any credit rating reports good or bad will be recorded against your personal credit references, not your Grandfather's.

The only time it would affect him is if you defaulted by failing to honour your agreement, in which case they could pursue him for the oustanding payments / balance.