Just to say, not all "ring and reserve" services are crap.
When I worked at Argos, I found their stock control to be exemplary. You know the little picking tickets that come attached to everything? They print on these if it is the last 1, 2 or 3 on the shelf. The person picking it has to verify the quantity shown. If the qty is wrong, they highlight this to a member of management who then alters the SOH figure on the store computer (which is the figure shown on the internet and on the surrounding stores computers).
This means the ring and reserve service, or reserve online, is rarely wrong at Argos. Clearly Halfords have everything on shelf, so are subject to shoplifting and mis-stocking etc. I wouldnt imagine heir stock-and-order procedures are quite as strict as Argos. Argos have the infrastructure in place to easily enable reservations, whereas Halfords dont.