Just thought I'd update this for anyone who helped me out with it:

I bought Twisted Metal again last night. Played it this morning. Initially, when trying to connect to online, I got the same old crap about connection errors, so I was pretty pissed.

I persevered with it for about 15 minutes and then something odd happened, my PSN got signed out. I went to check if my router had gone down, which it hadn't. I quit the game, logged back into PSN and then restarted it. Once I got back into the game, I had absolutely no problem getting into any servers. Joined every one I tried first time.

It was an odd series of events, but the online seems to be working 100% for me now. So thanks for the replies about that, you were all right!

Haha phew, I thought I had screwed you over at first xD Would have felt terrible :*