I need to replace some of the foam in the driver's seat base because my !Removed! hurts.

What do I have to be careful of regarding sudden airbag deployment when I unplug the electrics?

Second question, the seatbelt sensor is iffy, so it often bings at me when I'm wearing my seat belt. Can I leave the sensors unplugged or will that affect the airbag?

Hello Simon,

You need to disconnect the battery and leave the car for half an hour before unplugging any thing to do with the seat. 
As a matter of interest, where are you getting your replacement foam base from? Not from Audi I take it, a c£345! 
The seat belt warning light is likely to be as a result of the saggy base, and you may find it will survive the replacement base.

Kind regards,

Gareth. 

I've bought some memory foam and was just going to cut it to shape. The seat belt bing stops if I push the seat belt further into the clasp.

I hope you have some luck with the memory foam Simon, since the original base itself isn’t very deep and easily loses its ‘resilience’. 
I’m going to replace mine with one from a passenger seat - not too different ( I hope) from the contour of the driver’s seat. 
Kind regards,

Gareth.