FYI, the coolant 'loss' on removing the thermostat was around 2.5 litres. About 500ml comes out of the rad hose with a further 2 litres from the block/expansion tank once you pull out the thermostat. Managed to catch around 2 litres using a jug wedged in the gap on the RH side of the engine. Best to filter it back in to the expansion tank through a gauze to remove any larger dirt particles which get washed in to the jug.
No further losses of coolant if you are doing the coolant temp sensor at the same time, unfortunately SEAT gave me the wrong one and I needed the car so that'll now be a seperate job, if required (I was only changing it as the coolant was going to be out, the thermostat was the broken part).