how to bypass a rover 600 key recoginiser?

i have a s-reg rover that i lost the keys for.
i managed to get a key cut, but now when i try to start the car a flashing green key symbol appears on the dash, that doesn't allow the engine to turn over.
i bought the car for £200 so i dont want to spend much because i just want to sell it for whatever i can get to retrieve costs