CSG2306 is wrong about the insurance situation. You need special insurance cover (and probably a taxi or private hire licence) if your car is used 'for hire or reward'. However, sharing the costs of travel does not constitute 'hire or reward', so it's perfectly legal.
As I indicated in my previous post, a half-share of weekly costs might only amount to about a tenner if the car has a small engine and it's driven by someone who has economy in mind. However, a more powerful car (or less economical driving) could take that half-share nearer to �15. That figure is also around (or possibly slightly less than) a half of what most rail operators would charge you for a week's travel over the same distance.
That means that LouLou13, Andy Boz, RugeleyBoy, FunkyMoped and I all seem to agree that an offer of �15 would be reasonable.
Chris