Thanks to all for their input and contributions. I really do appreciate you taking the time to reply to my numpty problems.
I've been waiting for a few things to arrive - some low resistance cable, and a signal wire for my Gate mosfet, and thought I'd do everything at the same time, but in the event, I got home today and thought, "The hell with it..." and changed the barrel over without drama. The hop unit was quite incredibly robust, and will change the hop rubber over for something at some stage.
I don't know whether the barrel will make a whole heap of difference or not. To be honest, the biggest benefit was getting it apart, grubbing around a bit, and then getting it all back together again working. That did wonders for my my knowledge and confidence, which is probably, on balance, a good thing...
Cheers all