I've a Spider, the battery is in the cubby hole behind the drivers seat. I pull the seat forward, fold myself in half and sit on the centre console.
It's a bit fiddly getting it through the hole, but you only need a 10mm spanner/socket for the terminals and a 13mm socket with an extention bar to remove the clamp down the front that holds the battery to the floor.
Couple of notes.
Remove the - terminal first, you don't want to be swinging on the + nut and catch the body with the - terminal still attached!
Careful not to catch the spanner on both terminals, it's a no brainer, but it's been done (by me), if you've just charged the battery, there's plenty of gas in it that'll explode.
Clean off the terminals with a wire brush and smear with Vasaline (petrolium jelly) NOT KY jelly, blackcurrant jelly or normal grease!
If you suspect a drain on the battery somewhere, like the head unit in the first post of this thread,
Connect up + terminal only as normal
Set a multimeter to read Amps (important to get this right, so read the meters instructions)
Make sure everything on the cars turned off, lights inside and out, is the door open? it has sill lights, so close the door, alarm, etc
Connect the meter between the - terminal and the - lead (you might trigger the alarm to reset at this point, so have the fob to hand and turn it off) take a reading.
0.02 to 0.04 amps should be about right.
When my head unit was draining because it wasn't on the switched live, I was reading 0.48 amps, but it dropped when I disconnected the head unit, so I knew it was that what was causing a problem.
If you suspect a charge problem (alternator),
Connect the battery up as normal.
Set your meter to DC volts (see meter instructions)
Connect meter to battery, Red to +. Black to -
Take a reading with the engine running (tick over) and no load (lights off, etc)
Somewhere between 13.6 volts and 14.4 volts is normal, if the battery is low it may read lower, but you've just had it out and charged it.
If you suspect the battery has a fault.
You need to take if for what referred to as a "drop test"
I'm suspect if you take it to Halfords or Kwik Fit, it'll fail, I don't know why, but they always seem to! So try a local auto electrican.
A good cheap battery is available from ***** Batteries of Rochdale, the trade on Ebay, I've used them on plenty of cars, ran winches off them etc and never had a problem.