Like many big firms, Telegraph Media Group has it's own postcode, separate from all others in the same street. Simply putting SW1W 0XF on the envelope would be good enough to get any mail to them, with their post room ensuring that competition entries get to the right place. Sometimes they use a box number to help those staff, at other times it seems that they don't bother:
http://www.theanswerb...s/Question841540.html
If you simply write '£500 Crossword' as part of the address it should get there.
Chris