UPDATE:
Firstly, thanks for all the replies and advice...
So, springs arrived today, had already printed off Urban Panzer's how-to on SLS replacement (which is excellent, by the way) so away i went.
Checked ALL connections first, no leaks.
Checked old bags, no signs of leaks...it was always the driver's side that dropped, with the other side trying to compensate.
Changed the bags over, like everyone says, it really is a doddle.
Didn't disconnect battery / ride height sensors etc as no need to.
Driver's window down, doors shut, switched on, took a couple of seconds to get it's bearings, then rose immediately, a LOT faster than it used to.
Danced about with joy for a bit, then took it over a load of speed bumps in the village, not a murmur, loads more comfortable, quieter and smoother. The old driver's side bag must have been absolutely knackered!
"But why didn't you check all connections first?" I hear you ask.
Because after reading loads about this subject I was gonna change the bags regardless. As Biglad says, they should be treated as a service item.
Sincere thanks again for the help and advice, and if anyone wants a second hand, known to be good (ish) bag for diagnostic purposes, it's free to the first taker.
