First Capital Connect will take you to London Bridge in about 45 minutes From there you can take the London Underground Jubilee line to the West end or walk to Tower Bridge, Tower of London or St Pauls Cathedral.

Take an ordinary train instead. It’ll be slower than the Gatwick Express, which is non-stop, but cheaper. You can look up times and fares at http://www.firstcapitalconnect.co.uk/Main.php?iCmsPageId=75 A single adult ticket at off-peak times will be about £12 rather than £15 on the Gatwick Express.

National Express run an even cheaper coach service – £14 return – but it will take an hour and a half each way so that will knock out more of your available time.

Finally there is easyBus which can be as little as £2 single fare if you book in advance. It goes to West Brompton Underground station though, so you then have a bit more travelling to do to get anywhere interesting.

Once in London, get day Travelcards for all of you as the single Underground fare in central London is £4 and this is massively expensive if you hop about a lot. A day Travelcard for Zones 1 and 2, which are the only zones you will need, is less than twice the single fare and allows any number of trips on buses or the Underground. The idea is to stop people buying single tickets as much as possible and ideally make them use an electronic Oystercard ticket instead – the £4 reduces to £1.80 if you use Oyster, but just for a short stopover getting four of those is pointless.

You can take either the Gatwick Express or the Southern Trains…