The staff at the Bristol Paint Centre have mixed many paint pots for me to match existing paint. I just bring along a small sample of what ever I am painting and they mix me just what I need. I would guess I have had approx 16 paint pots mixed and only ever had one that wasn't quite right, however they fixed it up for me the next day quickly with no charge.
I will always buy my paint from this store.

Recently re decorated a retail shop. John matched the paints required from sometimes obscure chip samples supplied by me.Nothing too much trouble for these guys.I even have one or two colours named after me!! Good service , good quality,good price!!