If you're in the UK, you'll want to dial:
00800 0246 9273
Whatever you do, for heaven's sake DO NOT DIAL THIS NUMBER FROM A MOBILE if you're calling from the UK!
00800 numbers are an international 'freephone' number but most mobile (and indeed some landline service providers) do not abide by the 'free' agreement. Calls to 00800 numbers can cost as much as 50p a minute, meaning the hour you spend on hold during this busy time will set you back £30.