If I can recieve a parcel on a Saturday morning at 9am when I ordered on a Friday in the afternoon from a company in the south of England to be delivered to a small village near Elgin in Scotland (Using guranteed next day delivery through Royal Mail) then why can't I do that with DPD ?
ELGIN is mainland so obviously DPD are not the best service that overclockers can offer as far as next day delivery.
RM have hundreds (thousands) of effective hub areas, thousands of potential drivers (and tens of thousands of delivery staff), dealing with millions of items a day via thousands of trucks a night, not to mention trains, and if need be aircraft.
Albeit mainly setup to deal with smaller/lighter/low value items.
RM can afford to get trucks heading between depots very early in the evening, because they'll be relatively full, and potentially multiple trucks/vans from the smaller hubs/post offices to the larger ones every evening, most couriers don't have the staff/quantity of items to do that effectively (and at a reasonable cost).
DPD and most couriers if lucky have 50-100 hub locations and a few hundred/thousand delivery staff, and are set to do it the most cost effective way possible (RM have to try and make sure the post arrives next day, cheaply even if it means they make a loss on it).
DPD seem to be the best compromise between reliability, cost and speed, RM can provide speed (generally) but for a lot of items are not cost effective (SD which is the best service they offer for expensive items does a jump from about £8 to nearly £20 the moment you go over 2kg in weight), but offers little in the way of standard/easy to sort compensation if something is lost.
IIRC The store has used about 4 different couriers over the past 10 years to try and get the best combination of cost/speed/reliability and pretty much every one has had the same problems with certain areas, and problems in some areas (although those areas vary by courier)
