Waiting to get ours back atm but we got them done through Novosail - http://www.novosail.com/en-NovoSail-textilecleaning/horsecovers.php. New to me but other livery who arrived in winter has used them before and really rates them (and gets everything else done to a good standard). They post out sacks and tags for cleaning and/or repair, collect the sacks of rugs for cleaning and then they're brought back. We got ours done for £9 each including reproofing and VAT but I believe we had to have a minimum of 20 to get that rate. They're not near Preston but we must be a good 150 miles away so guessing they work nationally.
ETA I needed a neck cover strap repairing but did stay away from that bit - apparently £25 per hour for repairs but think they'd ok this in advance with owner.