The legal pack for auction should be viewable at the estate agents or vendor's solicitor from at least 14 days before the auction (and in some cases as a .pdf download subject to revision).
Make time to view it, make notes - look up and quary any points and view it again.
I bought a plot privately immediately after a collective auction sale, it was a subsequent option adjacent to a larger plot and the buyer of that declined the option so it went up prepared as a seperate lot. I didnt get a bid in as it was just over my limit as the auctioneer stalled for bids and turned out not to have met reserve and possibly one genuine bidder up to that point and was withdrawn.
I hung around until the room cleared and the auctioneer approached the last bidder to invite an offer over the reserve, he turned to me as he'd seen me the week before going over the legal pack in the office and asked my why I hadn't bid. I told him the negative aspects I considered regarding the plot and the other guy's wife dragged him away at that point - I bought it for £1000 less than the stalled price at withdrawal with agreement from the vendor 10 minutes later.
Completion was 13 days later, as all the major legal stuff is prepared prior to auction and my solicitor was embarrassed to charge me nearly £300 as he had so little to do.
My other plot bought privately through an agent, dragged on for about 4 months before the final Land Registry return came back confirmed, compared to the handiness of an auction preparation.
Most of that time was the absence of a prepared pack and a bit of to & fro over detail with the vendor.
(In fairness, the vendor made an effort to confirm, line up and measure boundary points during an on site visit prior to paying deposit).
For larger plots, particularly if previous use has been mixed / varied, a Groundsure estate survey might be beneficial (around £200)
http://www.groundsure.com/products/residential/groundsure-estateA word of caution, an unrelated local property that was advertised for a collective sale appeared 'attractive' until I viewed the digitised legal pack - some major points missing were an expired lease on a building still being used by a 3rd party that had expired 20 years previously, an enforcement notice on another building on the plot and access over common land owned by someone else, as well as being in a flood-plain.
Most of that information was obtained in minutes by downloading the title deeds and flood risk map for less than £10 off the official Land Registry website - fortunately local knowledge lead me to believe that deal seemed too good to be true, so I checked and queried it with the agent (they suggested the seller wasn't aware, but had to revise the legal pack prior to sale).