Rory Cellan-Jones walking through Paddington station while viewed through train window
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Mapping Britain's 3G mobile data networks: London

A BBC crowd-sourcing survey to map the UK's mobile phone networks has revealed the parts of Britain where mobile phones can access the fastest data speeds.

Users of more than 44,000 phones downloaded an app to measure mobile phone signals over a month.

The software helped build an interactive map which lets people check reception at specific postcodes.

Rory Cellan-Jones took the train from Cardiff to London to test data speeds along the route, ending in Paddington.

There he met mobile expert Windsor Holden, who is pessimistic about communities in 3G "notspots" receiving high speed signals in the near future.

Go to next video: Mapping Britain's 3G: Cardiff