Wish me luck then. Having managed to completely build the bike from scratch (I'm a bit of a numpty, but learning) I steered clear of hydraulics for just this sort of reason, I guess I need to bit the bullet, if I managed it once I'll be fine. The piston didn't pop out, it stayed in place but leaked oil from one area, so hopefully everything will be fine.
On a related issue, CRC are selling LX calipers for £19.99. They're list as rear 160mm IS. I'm assuming that, like my old Avid mechanicals, the calipers are the same and it's the adapters that count. Could I simply fit a rear 160mm caliper to my current front 180mm adapter and use that?