I have a 57 plate Fiesta ST 2.0L with a Mountune MP185bhp. I'm considering taking the back seats out and putting a roll cage in. Is there a lot of power to gain by doing this and has anyone else done it to there?

I guess the only gain would be if the roll cage is lighter than the rear seats

Have you done this conversion yourself? Do you know where I could get a decent roll cage from

You'll gain very little weight wise, what you need to think is what cage do you want?

Bolted cage - easy to fit and remove, adds a little rigidity, generally a cost effective way to go

Welded cage - need a professional to fit, adds a lot of rigidity, generally costly, non removable (unless you have someone who can do it well but generally seen as not worth while

How much cage are we talking about? Just the back for looks? Or a full down to the dash cage? Or door bars and such?

All depends on what you want, generally the more cage you get and the better job you get, the better the car will feel but the higher the cost. I have one in my Mini that cost £450 for a full bolt in cage and I'm happy with the results.