Harry Shrimpton

Hi all, I have a fiesta mk 5 zetec s as stated but have had a problem pop up on me. I was driving her around the block because of of previous troubles with a maf sensor which I replaced a week ago and it drove nice everything fine until I parked it and turned off the engine. The dash lights stayed on which my friend noticed as I got out, I got back in went to start it and nothing, dash lights stayed on and car didn't attempt to start. There's a red light flashing like mad so I think it must be immobiliser related. I used the spare key today which set off the alarm but turned it off and was able to drive it.
Any help would be appreciated,
Thanks Harry