There is no credited writer. Majority of the film is improvised.
The two Michael Caine lines that Steve Coogan and Rob Brydon repeat again and again - "you were only supposed to blow the bloody doors off!" and "she was only 15" - are from The Italian Job (1969) and Get Carter (1971) respectively... and actually she was "only 16".
Early in the film, the car stereo plays "Atmosphere" by Joy Division, which Steve Coogan explains he has chosen as the "soundtrack" for their trip, Though neither the song title nor the band are mentioned, in 24 Hour Party People (2002), Coogan played Tony Wilson, the producer who signed Joy Division to his Factory Records label.