I appear to have gotten stuck, before I found out the Enclave base was near the gas station I killed the guy in the purple robes (Chris), and so cannot talk to him about joining the Enclave or whatever.
However, there is a secret entrance inside the building, under a manhole cover iirc. Only problem is, the gas station door will not open at all, maybe it's locked but I don't think it's supposed to be and I can't remember if you actually get a message saying 'this door is locked' or not (I don't in this case). Without being able to go through this door I can't finish the game as far as I know, also I tried the other entrance in the woods with the Deathclaw (whom I set free). There is a door in there that does not respond to the blue keycard so I don't think I can get in that way.
Thanks in advance if anyone can help me,I love this game and I'd like to be able to continue :)

Do you mean the trapdoor/drain thing next to a big rock with a little stream trickling down it? I've been there, set the Deathclaw free using the blue keycard to open the door. Thing is, the door on the other side of the room doesn't seem to use the blue key to open, I can see the elevator right behind it but can't open the door.

No. Prator is referring to an actual path through the woods that leads northeast. The path is marked with little stones or dots or whatever they are.
But like he warned you. You'll have to shoot your way in. It's fun if you have the firepower.

Fixed my problem :D I wondered why Chris' dead body kept talking to me so I went into combat mode and realised he wasn't dead, just incapacitated. After killing him I was able to use the door to 'sneak' into the base. Thanks for offering alternatives though guys, I appreciate it :)

Seems I'm late to the party, but you could have just picked the lock on the door; I always do that, and I don't have to bother with Crhis at all that way ;-).