I've been buying online from Robinsons for years (not lucky enough to live near any of their store unfortunately), and have always been impressed with the range of goods, prices and customer service.
They regularly beat similar companies on prices, and their new Riders' Rewards scheme is great - every purchase earns points which count towards future purchases. This is in addition to regular money-off offers.
Delivery is usually pretty speedy and communication is good. I've yet to have to chase any orders up, and I've placed a lot of orders over the years!
I have 2 horses myself and get through a hell of alot of supplements espcially in winter. Robinsons were brilliant and made sure everything was sent out to me asap. Never had a problem, only thing I would say is make sure that you really want any big items as their return system can be a bit of a mindfield.
My daughter is into horse riding and Robinsons is a great site for horse related equipment and clothing. The prices are pretty good and the delivery time is amazing (often the next day). My only gripe is that some of the clothing is by age and other clothing by cm. Overall a very good experience