The Daily Telegraph are doing 2 tickets for the price of 1 for the preview day on the 24th
Go to the
Telegraph Motoring website for more details.
Basically you call up the box office hotline - 0870 906 2002, say you want tickets for the 24th - quote 'The Daily Telegraph reader offer', and they'll charge you £25 + £1 transaction fee for the 2 tickets. Not sure how long the offer lasts for, but I booked mine last night.