If you're using a Wilkinson/Youngs pressure barrel, I've never ever been able to achieve any kind of carbonation in one of them. I believe it has to do with the pressure relief valve that's built in to the cap; it just lets all the CO2 out, thus none of it is absorbed into the beer. You can buy a cap from Wilko that has a CO2 dispenser built in but they're around £18-20. Finally, beer conditions MUCH faster in bottles than it does in barrels!
Today, I only use my barrels for pale ale/very lightly carbonated bitters and I bottle/crown cap *everything* else. You can get an amazingly good head and carbonation by bottling. If you're doing beer kits, I'd recommend using dried malt extract instead of granulated sugar in your brew. I think it makes a big difference in the head and taste of my kits (though others say they don't notice a difference). Plain old Tate & Lyle is just fine for priming though.
Lucky Eddie got it right! Now, if you'll excuse me, I have to go stare at the air locks on my FV's for a few hours.
Good luck!